Catalyst for oxidation or ammoxidation

ABSTRACT

Disclosed is a catalyst for use in catalytic oxidation or ammoxidation of propane or isobutane in the gaseous phase, which comprises an oxide containing, in specific atomic ratios, molybdenum (Mo), vanadium (V), niobium (Nb) and antimony (Sb), wherein the oxide catalyst has a reduction ratio of from 8 to 12% and a specific surface area of from 5 to 30 m 2 /g. Also disclosed is a process for efficiently producing this catalyst.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a catalyst for use in catalyticoxidation or ammoxidation of propane or isobutane in the gaseous phase.More particularly, the present invention is concerned with an oxidecatalyst for oxidation or ammoxidation, which comprises an oxidecontaining, in specific atomic ratios, molybdenum (Mo), vanadium (V),niobium (Nb) and antimony (Sb), wherein the oxide catalyst has areduction ratio of from 8 to 12% and a specific surface area of from 5to 30 m²/g. The present invention is also concerned with a process forefficiently producing this catalyst. The catalyst of the presentinvention is advantageous not only in that the selectivity for and yieldof the desired product in the oxidation or ammoxidation are high, butalso in that the catalyst exhibits only a small lowering of the yield ofthe desired product even in a long reaction time. Therefore, when thecatalyst of the present invention is used for performing a catalyticoxidation or ammoxidation of propane or isobutane in the gaseous phase,an unsaturated carboxylic acid or an unsaturated nitrile (namely,(meth)acrylic acid or (meth)acrylonitrile) can be produced stably inhigh yield for a long period of time. Further, since the catalyst of thepresent invention exhibits only a small lowering of the yield with thepassage of reaction time, the catalyst of the present invention is alsoadvantageous in that, when a molybdenum compound is added to thecatalytic oxidation or ammoxidation reaction system as conventionallypracticed in the art for the purpose of maintaining a high yield bypreventing a catalyst degradation caused by the volatilization orescaping of molybdenum from the catalyst, the amount of molybdenumcompound added and the frequency of addition of molybdenum compound canbe decreased, as compared to the case of the use of conventionalcatalysts, so that the reaction can be performed economically. Inaddition, the catalyst of the present invention is advantageous in thata moderate catalyst activity can be exhibited, and hence there can beprevented problems that too large an amount of catalyst is required forthe reaction, thus causing too heavy a load on the reactor and that theheat of reaction generated becomes too large, rendering it impossible toeffect a satisfactory heat removal from the reaction system.

2. Prior Art

Conventionally, there have been well known a process for producing(meth)acrylonitrile by ammoxidation of propylene or isobutylene, and aprocess for producing (meth)acrylic acid by oxidation of propylene orisobutylene. Recently, as substitutes for such processes for theoxidation and ammoxidation of propylene or isobutylene, attention hasbeen attracted to a process for producing (meth)acrylonitrile by acatalytic ammoxidation of propane or isobutane in the gaseous phase, anda process for producing (meth)acrylic acid by a catalytic oxidation ofpropane or isobutane.

As catalysts which can be used for increasing the selectivity and yieldin the reactions used in these processes, a number of oxide catalystscontaining molybdenum, vanadium, niobium and antimony have beenproposed.

For example, various catalyst compositions intended for producing(meth)acrylonitrile or (meth)acrylic acid with high selectivity and inhigh yield are disclosed in various patent documents, such as UnexaminedJapanese Patent Application Laid-Open Specification Nos. Hei 9-157241(corresponding to U.S. Pat. No. 5,750,760 and EP 767164B1), Hei10-45664, and 2002-239382 (corresponding to U.S. Pat. No. 7,109,144 B2,US 2002/0115879 A1 and US 2006/0252954 A1).

Further, there are prior art documents which disclose the averagevalence of the component elements of a catalyst or disclose the atomicratio of oxygen in a catalyst formulation. For example, UnexaminedJapanese Patent Application Laid-Open Specification No. 2002-301373 hasa description about the average valence of the component elements (otherthan the carrier) of a catalyst. Specifically, this patent documentstates that the average valence is generally from 4 to less than 6,preferably from 4.5 to 5.9, more preferably from 5 to 5.8. UnexaminedJapanese Patent Application Laid-Open Specification No. 2003-24790(corresponding to U.S. Patent Application Publication No. US2002/0183548 A1 and EP 1254708A2) states that the representative atomicratio of oxygen in a catalyst formulation is from 3 to 4.7, relative tomolybdenum.

However, the catalysts (containing molybdenum, vanadium, niobium andantimony) disclosed in these patent documents are still unsatisfactorywith respect to performance and hence cannot be commerciallyadvantageously employed.

There are known various methods for producing catalysts which canincrease the selectivity for and yield of the desired product inoxidation or ammoxidation. For example, such methods for producingcatalysts are disclosed in Unexamined Japanese Patent ApplicationLaid-Open Specification No. Hei 10-28862, EP 895809A1, UnexaminedJapanese Patent Application Laid-Open Specification Nos. 2001-58827,2002-301373, 2002-316052 and 2003-24790 (corresponding to U.S. PatentApplication Publication No. US 2002/0183548 A1 and EP 1254708A2).

Especially, there are prior art documents which provide a teaching abouta calcination method used in a method for producing a catalyst which canincrease the selectivity for and yield of the desired product inoxidation or ammoxidation. For example, Unexamined Japanese PatentApplication Laid-Open Specification No. Hei 9-157241 (corresponding toU.S. Pat. No. 5,750,760 and EP 767164B1) states that the calcination maybe performed in an oxygen-containing atmosphere, but is preferablyperformed in an oxygen-free atmosphere. Unexamined Japanese PatentApplication Laid-Open Specification No. Hei 10-28862 states that thecalcination may be conducted using either a fluidized-bed kiln or arotary kiln or using these kilns in combination. Unexamined JapanesePatent Application Laid-Open Specification No. Hei 10-45664 states that,prior to conducting the calcination, the catalyst precursor may besubjected to a thermal decomposition in the air to remove the most ofvolatile components from the catalyst precursor. Further, UnexaminedJapanese Patent Application Laid-Open Specification No. 2002-316052states that, in the case of a continuous calcination, the calcination isperformed while supplying an inert gas at a flow rate of from 500 to10,000 N liters per 1 kg of the supplied catalyst precursor, therebyeffecting a thermal decomposition of the catalyst precursor.

However, with respect to the calcination methods employed in these priorart documents, there has not yet been found an important factor greatlyinfluencing the selectivity for and yield of the desired product whichare exhibited by the obtained catalyst. Therefore, the selectivity forand yield of the desired product which are exhibited by the catalystsproduced by employing the conventional methods are not satisfactory fromthe commercial viewpoint.

A catalyst for commercial use not only needs to exhibit a high yield atthe early stage of the reaction, but also needs to maintain the yieldeven when the reaction is performed for a long time (specifically, 1,500hours or more). When the yield cannot be maintained for a long reactiontime, it is conceivable to take the deactivated catalyst out of thereactor and feed a fresh catalyst into the reactor; however, suchreplacement of the deactivated catalyst by a fresh catalyst has aproblem in that the replacement operation is cumbersome, hinders thecontinuous operation of the reactor and is also disadvantageous from theeconomic viewpoint. It is also conceivable to take a measure in whichthe degraded catalyst is taken out of the reactor and subjected to aregeneration operation to thereby obtain a regenerated catalyst, whichis then returned to the reactor; however, this measure poses a problemin that the regeneration operation takes a long time and needs acomplicated regeneration equipment and/or that a satisfactoryregeneration of the catalyst cannot be achieved. Accordingly, there hasbeen a demand for an excellent catalyst which exhibits only a smalllowering of the yield of the desired product in a catalytic oxidation orammoxidation reaction. For example, Unexamined Japanese PatentApplication Laid-Open Specification No. 2002-239382 (corresponding toU.S. Pat. No. 7,109,144 B2, US 2002/0115879 A1 and US 2006/0252954 A1)discloses a catalyst which maintains the selectivity at almost the samelevel, although the selectivity can be maintained only for a relativelyshort reaction time of about 1,000 hours. However, this catalystexhibits a low activity and hence a low conversion of propane fed;therefore, when this catalyst is used in a one pass mode of reaction,the yield of the desired product is not high. When a catalyst exhibits alow conversion of propane, it is conceivable to take a measure in whichthe unreacted propane is separated and recovered from the gas flowingout of the reactor and recycled to the reactor; however, this measure isdisadvantageous in that the process of separation, recovery andrecycling of the unreacted propane requires a large scale equipment.Unexamined Japanese Patent Application Laid-Open Specification No. Hei11-169716 discloses a catalyst which maintains the yield at almost thesame level, although the yield can be maintained only for a relativelyshort reaction time of about 1,300 hours. However, the working examplesof this patent document employ a catalyst which contains tellurium butno antimony, and there is no specific description of a catalystcontaining molybdenum, vanadium, niobium and antimony. Further, when acatalyst containing tellurium is used in a commercial scale reaction, aproblem tends to arise in that the tellurium volatilizes and escapesfrom the catalyst with the passage of reaction time, thus destabilizingthe reaction and rendering it difficult to commercially perform thereaction for a long time. In Unexamined Japanese Patent ApplicationLaid-Open Specification No. Hei 2-2877 (corresponding to U.S. Patent No.4,784,979 and EP 320124A), there is a description of a redox reaction ofantimony and vanadium, but there is no technical concept of controllingthe reduction ratio of a catalyst. In addition, it is presumed that thereduction ratio of a catalyst which is produced under the catalystproduction conditions used in the working examples of this patentdocument would be much lower than 8%.

On the other hand, with respect to a catalyst containing molybdenum,there are cases where the catalyst is degraded by the volatilization andescaping of molybdenum from the catalyst, although the degree of thedegradation is small, as compared to the degradation caused by thevolatilization and escaping of tellurium. For preventing thisdegradation, there has conventionally been known a method in which amolybdenum compound is added to the reactor during the reaction.

For example, Unexamined Japanese Patent Application Laid-OpenSpecification No. 2001-213855 discloses a process for producing anunsaturated nitrile stably in a high yield by using a catalystcontaining molybdenum, vanadium, niobium and antimony, wherein theprocess involves a step of adding to the reaction system a compensativecompound comprising at least one compound selected from the groupconsisting of a tellurium compound and a molybdenum compound. In thispatent document, the amount of the compensative compound is described tobe such that the weight ratio of the compensative compound to thecatalyst is equal to or less than 0.1/1, preferably equal to or lessthan 0.02/1. In Example 2 of this patent document, it is described thata reaction is performed for 53 hours in total, wherein during thereaction, both a tellurium compound and a molybdenum compound aresimultaneously added to the reaction system, each in an amount of 0.1 g,relative to 45 g of the catalyst (namely, the weight ratio of eachcompound to the catalyst is 0.0022/1). This means that the amount ofmolybdenum compound which added to the reaction system per hour of thereaction time is such that the weight ratio of the molybdenum compoundto the catalyst is as large as 0.000042/1; that is, a large amount ofmolybdenum compound is added to the reaction system. In the case where amolybdenum compound is added to a reaction system for the purpose ofmaintaining the yield, when the molybdenum compound is added in a largeamount, there occur problems not only in that the cost of the molybdenumcompound becomes large, which is economically disadvantageous, but alsoin that, when the reaction is conducted using a fluidized-bed reactor,the molybdenum compound added adheres to the heat removal coil in thereactor, thus hindering the transfer of heat to the heat removal coiland rendering it impossible to perform a stable reaction. Therefore,there has been a demand for a catalyst which has an advantage in that,in performing the conventional practice of adding a molybdenum compoundto the reaction system, the amount of molybdenum compound added and thefrequency of addition of molybdenum compound can be decreased to a levelas low as possible.

For producing a desired product stably and economically on a commercialscale, it is especially important to maintain the yield of the desiredproduct at a high level for more than 1,500 hours from the start of thereaction. In this respect, there has been a demand for a catalyst whichcan exhibit a performance such that, even more than 1,500 hours afterthe start of the reaction, a high yield can be maintained by theaddition of only a small amount of molybdenum compound to the reactionsystem. However, there has not yet been known an excellent catalystwhich exhibits only a small lowering of the yield of the desiredproduct, thereby enabling the maintenance of a high yield by theaddition of only a small amount of molybdenum compound during thereaction.

Further, it should be noted that, with respect to a catalyst which iscommercially used in a catalytic oxidation or ammoxidation of propane orisobutane in the gaseous phase, the catalyst is required to exhibit animportant performance such that, in addition to high yield and highstability of the yield with the passage of reaction time, the catalystexhibits a moderate activity. In general, when a catalyst having lowactivity is used in a catalytic oxidation or ammoxidation, the catalystis used in an increased amount for the purpose of obtaining a desiredconversion of a raw material used. In such a case, however, when theactivity of the catalyst is too low, disadvantages occur not only inthat too large an amount of catalyst is necessary, but also in that theload on the reactor becomes large, and the size of the reactor needs tobe increased.

In the case of a catalyst having too low an activity, it is naturallyconceivable to take a measure in which the catalytic activity isincreased by raising the reaction temperature; however, this measureposes a problem not only in that, when the reaction temperature israised to a level which is higher than an appropriate temperature, theyield of the desired product is decreased, but also in that, in the caseof an ammoxidation reaction, the ammonium used as a raw material iswastefully burnt without being used for producing the desired product.Also, the use of too high a reaction temperature is undesired because ofthe occurrence of adverse effects on the material of the reactor.

On the other hand, when a catalyst having too high an activity is usedin a catalytic oxidation or ammoxidation, there is a problem in that theconversion of a raw material used is increased too much, leading to alowering of the yield of the desired product and a generation of toogreat an amount of heat of reaction. Therefore, it is conceivable totake a measure in which the amount of catalyst used is decreased.However, this measure poses the following problem. With respect to afluidized-bed reactor which is used for performing a catalytic oxidationor ammoxidation on a commercial scale, the fluidized-bed reactor isequipped with a heat removal coil designed for removing the heat ofreaction generated during the oxidation or ammoxidation. In this case,when the amount of the catalyst used is decreased in an attempt toprevent the adverse effects of too high a catalytic activity, thedecrease in the amount of the catalyst results in a decrease in thecontact area between the catalyst and the heat removal coil, thusrendering it impossible to effect a satisfactory heat removal andcontinue the operation of the reactor. There is a further problem inthat the amount of the raw material gas per unit weight of the catalystbecomes too large, which tends to cause a degradation of the catalyst.It is conceivable to take a measure in which the catalytic activity isdecreased by lowering the reaction temperature; however, this measureposes a problem in that the selectivity for the desired product isdecreased.

As seen from the above, there has not yet been known a catalyst which isadvantageous not only in that the selectivity for and yield of thedesired product in the oxidation or ammoxidation are high, but also inthat the catalyst exhibits only a small lowering of the yield of thedesired product even in a long reaction time, and the yield of thedesired product can be easily maintained at a high level for a longreaction time, while exhibiting a moderate catalyst activity.

SUMMARY OF THE INVENTION

In this situation, the present inventors have made extensive andintensive studies with a view toward solving above-mentioned problems ofthe prior art, specifically toward developing an excellent oxidecatalyst containing molybdenum, vanadium, niobium and antimony, which isfor use in a catalytic oxidation or ammoxidation of propane or isobutanein the gaseous phase, and toward developing a process for producing suchexcellent catalyst. As a result, it has unexpectedly been found thatthis objective can be attained by a catalyst which comprises an oxidecontaining, in specific atomic ratios, molybdenum (Mo), vanadium (V),niobium (Nb) and antimony (Sb), wherein the oxide catalyst has areduction ratio of from 8 to 12% and a specific surface area of from 5to 30 m²/g. That is, it has unexpectedly been found that such catalystexhibits excellent properties: that a moderate catalyst activity can beexhibited; that the results of the reaction (selectivity for and yieldof the desired product) are satisfactory; and that the catalyst exhibitsonly a small lowering of the yield of the desired product even in a longreaction time, and therefore a lowering of the yield can be easilyprevented by the addition of only a small amount of molybdenum compoundand with less frequency of the addition even in a long reaction time.The present inventors have also found that this catalyst can beefficiently produced by a catalyst production process employing specificconditions for calcination. Based on these findings, the presentinvention has been completed.

Accordingly, it is an object of the present invention to provide acatalyst for use in catalytic oxidation or ammoxidation of propane orisobutane in the gaseous phase, which comprises an oxide containing, inspecific atomic ratios, molybdenum (Mo), vanadium (V), niobium (Nb) andantimony (Sb), wherein the catalyst is advantageous not only in that thecatalyst exhibits a high yield of the desired product and is capable ofmaintaining the yield at a high level for a long reaction time by addingto the reaction system only a small amount of molybdenum compound andwith less frequency of the addition, but also in that a moderatecatalyst activity can be exhibited.

It is another object of the present invention to provide a process forefficiently producing the above-mentioned catalyst.

It is still another object of the present invention to provide a processfor producing an unsaturated carboxylic acid or an unsaturated nitrile(namely, (meth)acrylic acid or (meth)acrylonitrile) by using theabove-mentioned catalyst.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect of the present invention, there is provided a catalyst foruse in catalytic oxidation or ammoxidation of propane or isobutane inthe gaseous phase, which comprises an oxide represented by the followingformula (1):Mo₁V_(a)Nb_(b)Sb_(c)O_(n)  (1)

wherein:

-   -   a, b, c and n are, respectively, the atomic ratios of vanadium        (V), niobium (Nb), antimony (Sb) and oxygen (O), relative to        molybdenum (Mo),        -   wherein:            0.1≦a≦1,            0.01≦b≦1,            0.01≦c≦1, and        -    n is the number of oxygen atoms required to satisfy the            valence requirements of the other component elements            present,    -   the catalyst having a reduction ratio of from 8 to 12% and a        specific surface area of from 5 to 30 m²/g,    -   the reduction ratio being represented by the following formula        (2):        reduction ratio (%)=((n ₀ −n)/n ₀)×100  (2)

wherein:

-   -   n is as defined for formula (1), and    -   n₀ is the number of oxygen atoms required when the other        component elements in the oxide of formula (1) respectively        exhibit the maximum oxidation numbers of the other component        elements.

In another aspect of the present invention, there is provided a processfor producing the above-mentioned catalyst, which comprises the stepsof:

providing an aqueous raw material mixture containing compounds ofmolybdenum, vanadium, niobium and antimony,

drying the aqueous raw material mixture to thereby obtain a driedcatalyst precursor, and

calcining the dried catalyst precursor under calcination conditionswherein the heating temperature of the dried catalyst precursor iscontinuously or intermittently elevated from a temperature which is lessthan 400° C. to a temperature which is in the range of from 550 to 700°C., wherein the calcination conditions are adjusted so that the catalystprecursor being calcined has a reduction ratio of from 8 to 12% when theheating temperature reaches 400° C., wherein the reduction ratio is asdefined above in connection with the above-mentioned catalyst,

thereby obtaining a catalyst having a reduction ratio of from 8 to 12%and a specific surface area of from 5 to 30 m²/g.

For easy understanding of the present invention, the essential featuresand various preferred embodiments of the present invention areenumerated below.

-   1. A catalyst for use in catalytic oxidation or ammoxidation of    propane or isobutane in the gaseous phase, which comprises an oxide    represented by the following formula (1):    Mo₁V_(a)Nb_(b)Sb_(c)O_(n)  (1)

wherein:

-   -   a, b, c and n are, respectively, the atomic ratios of vanadium        (V), niobium (Nb), antimony (Sb) and oxygen (O), relative to        molybdenum (Mo),        -   wherein:            0.1≦a≦1,            0.01≦b≦1,            0.01≦c≦1, and        -    n is the number of oxygen atoms required to satisfy the            valence requirements of the other component elements            present,    -   the catalyst having a reduction ratio of from 8 to 12% and a        specific surface area of from 5 to 30 m²/g,    -   the reduction ratio being represented by the following formula        (2):        reduction ratio (%)=((n ₀ −n)/n ₀)×100  (2)

wherein:

-   -   n is as defined for formula (1), and    -   n₀ is the number of oxygen atoms required when the other        component elements in the oxide of formula (1) respectively        exhibit the maximum oxidation numbers of the other component        elements.

-   2. The catalyst according to item 1 above, wherein a, b and c in    formula (1) are as follows:    0.1≦a≦0.3,    0.05≦b≦0.2,    0.1≦c≦0.3.

-   3. The catalyst according to item 1 or 2 above, which further    comprises a silica carrier having supported thereon the oxide,    wherein the silica carrier is present in an amount of from 20 to 60%    by weight in terms of SiO₂, based on the total weight of the oxide    and the silica carrier.

-   4. The catalyst according to any one of items 1 to 3 above, wherein    no in formula (2) is from 4 to 5.

-   5. A process for producing the catalyst of item 1 above, which    comprises the steps of:

providing an aqueous raw material mixture containing compounds ofmolybdenum, vanadium, niobium and antimony,

drying the aqueous raw material mixture to thereby obtain a driedcatalyst precursor, and

calcining the dried catalyst precursor under calcination conditionswherein the heating temperature of the dried catalyst precursor iscontinuously or intermittently elevated from a temperature which is lessthan 400° C. to a temperature which is in the range of from 550 to 700°C., wherein the calcination conditions are adjusted so that the catalystprecursor being calcined has a reduction ratio of from 8 to 12% when theheating temperature reaches 400° C., wherein the reduction ratio is asdefined in item 1 above,

thereby obtaining a catalyst having a reduction ratio of from 8 to 12%and a specific surface area of from 5 to 30 m²/g.

-   6. The process according to item 5 above, wherein the aqueous raw    material mixture is obtained by mixing an aqueous mixture (A)    containing compounds of molybdenum, vanadium and antimony with an    aqueous liquid (B) containing a niobium compound.-   7. The process according to item 6 above, wherein the aqueous    mixture (A) is obtained by heating, at 50° C. or more, compounds of    molybdenum, vanadium and antimony in an aqueous solvent.-   8. The process according to item 7 above, wherein, after the    heating, hydrogen peroxide is added to the aqueous mixture (A).-   9. The process according to item 8 above, wherein the amount of the    hydrogen peroxide is such that the molar ratio (H₂O₂/Sb molar ratio)    of the hydrogen peroxide to the antimony compound in terms of    antimony is in the range of from 0.01 to 20.-   10. The process according to item 6 above, wherein the aqueous    liquid (B) contains a dicarboxylic acid in addition to the niobium    compound, wherein the molar ratio (dicarboxylic acid/Nb molar ratio)    of the dicarboxylic acid to the niobium compound in terms of niobium    is in the range of from 1 to 4.-   11. The process according to item 6 or 10 above, wherein at least a    part of the aqueous liquid (B) containing a niobium compound is used    in the form of a mixture thereof with hydrogen peroxide.-   12. The process according to item 11 above, wherein the amount of    the hydrogen peroxide is such that the molar ratio (H₂O₂/Nb molar    ratio) of the hydrogen peroxide to the niobium compound in terms of    niobium is in the range of from 0.5 to 20.-   13. The process according to item 6 or 10 above, wherein at least a    part of the aqueous liquid (B) containing a niobium compound is used    in the form of a mixture thereof with hydrogen peroxide and an    antimony compound.-   14. The process according to item 13 above, wherein:

the amount of the hydrogen peroxide is such that the molar ratio(H₂O₂/Nb molar ratio) of the hydrogen peroxide to the niobium compoundin terms of niobium is in the range of from 0.5 to 20, and

the amount of the antimony compound mixed with the at least a part ofthe aqueous liquid (B) and the hydrogen peroxide is such that the molarratio (Sb/Nb molar ratio) of the antimony compound in terms of antimonyto the niobium compound in terms of niobium is not more than 5.

-   15. The process according to item 5 above, wherein at least a part    of the calcination is performed in an atmosphere of an inert gas,    wherein:

when the calcination is performed in a batchwise manner, the inert gasis supplied at a flow rate of not less than 50 N liters/hour/kg of thedried catalyst precursor, and

when the calcination is performed in a continuous manner, the inert gasis supplied at a flow rate of not less than 50 N liters/kg of the driedcatalyst precursor.

-   16. The process according to item 5 or 15 above, wherein the    calcination comprises a preliminary calcination and a final    calcination, wherein the preliminary calcination is performed at a    temperature in the range of from 250 to 400° C. and the final    calcination is performed at a temperature in the range of from 550    to 700° C.-   17. The process according to item 5, 15 or 16 above, wherein, during    the calcination, an oxidant or a reductant is added to an atmosphere    in which the calcination is performed, so as to cause the catalyst    precursor being calcined to have a reduction ratio of from 8 to 12%    when the heating temperature reaches 400° C.-   18. The process according to item 17 above, wherein the oxidant is    oxygen gas.-   19. The process according to item 17 above, wherein the reductant is    ammonia.-   20. A process for producing acrylic acid or methacrylic acid, which    comprises reacting propane or isobutane with molecular oxygen in the    gaseous phase in the presence of the catalyst of item 1 above.-   21. A process for producing acrylonitrile or methacrylonitrile,    which comprises reacting propane or isobutane with ammonia and    molecular oxygen in the gaseous phase in the presence of the    catalyst of item 1 above.

Hereinbelow, the present invention is described in detail.

The catalyst of the present invention comprises molybdenum, vanadium,niobium and antimony as the component elements thereof.

The catalyst of the present invention comprises an oxide represented bythe following formula (1):Mo₁V_(a)Nb_(b)Sb_(c)O_(n)  (1)

wherein:

-   -   a, b, c and n are, respectively, the atomic ratios of vanadium        (V), niobium (Nb), antimony (Sb) and oxygen (O), relative to        molybdenum (Mo),        -   wherein:            0.1≦a≦1,            0.01≦b≦1,            0.01≦c≦1, and        -    n is the number of oxygen atoms required to satisfy the            valence requirements of the other component elements            present.

In the above-mentioned formula (1), it is preferred that a, b and c(respectively representing the atomic ratios of vanadium (V), niobium(Nb) and antimony (Sb), relative to molybdenum (Mo)) are as follows:0.1≦a≦0.5, 0.01≦b≦0.5, 0.01≦c≦0.5, more advantageously 0.1≦a≦0.3,0.05≦b≦0.2, 0.1≦c≦0.3.

When the reaction mode is a fluidized-bed reaction, the catalyst isrequired to have high strength. Therefore, in such a case, it ispreferred that the catalyst of the present invention is used in a formsuch that the oxide of formula (1) is supported on a sufficient amountof a silica carrier for providing satisfactory strength.

When the catalyst of the present invention further comprises a silicacarrier, it is preferred that the silica carrier has supported thereonthe oxide, wherein the silica carrier is present in an amount of from 20to 60% by weight, more advantageously from 30 to 50% by weight, in termsof SiO₂, based on the total weight of the oxide and the silica carrier.

When the amount of the silica carrier in the catalyst is smaller than20% by weight, the strength of the catalyst becomes unsatisfactory, sothat the catalyst is likely to become powdery during the reaction andescape from the reactor, thus posing problems in that it becomesimpossible to perform a stable reaction on a commercial scale and thateconomic disadvantages are caused as it is required to supply anadditional amount of the catalyst to compensate for the catalyst loss.

On the other hand, when the amount of the silica carrier in the catalystis larger than 60% by weight, there cannot be obtained a satisfactoryactivity, and hence the amount of catalyst necessary for the reaction isincreased. Especially, in the case where the reaction mode is afluidized-bed reaction, when the amount of the silica carrier in thecatalyst is larger than 60% by weight, the specific gravity of thecatalyst becomes too small, rendering it difficult to achieve anexcellent flow.

The catalyst of the present invention has a reduction ratio of from 8 to12%, preferably from 9 to 11%. When the reduction ratio is lower than8%, the selectivity for the desired product becomes low, and further,the activity of the catalyst becomes extremely low. On the other hand,when the reduction ratio is higher than 12%, the activity of thecatalyst becomes low, and further, the selectivity for the desiredproduct becomes extremely low.

In the present invention, the reduction ratio is represented by thefollowing formula (2):reduction ratio (%)=((n ₀ −n)/n ₀)×100  (2)

wherein:

-   -   n is as defined for formula (1), and    -   n₀ is the number of oxygen atoms required when the other        component elements in the oxide of formula (1) respectively        exhibit the maximum oxidation numbers of the other component        elements.    -   n₀ in formula (2) can be obtained by calculation from the ratios        of the component elements contained in the raw materials        employed. The maximum oxidation numbers of the component        elements are as follows: molybdenum has a maximum oxidation        number of 6; vanadium has a maximum oxidation number of 5,        niobium has a maximum oxidation number of 5; and antimony has a        maximum oxidation number of 5. When the catalyst contains a        component element (e.g., tungsten) other than molybdenum,        vanadium, niobium, antimony and oxygen, n and n₀ are determined        so that the valence of the other component element and the        atomic ratio of the other component element, relative to        molybdenum, are reflected.

It is preferred that n₀ in formula (2) is from 4 to 5.

In the present invention, the specific surface area of the catalyst ismeasured by the BET method, namely the method based on the BETadsorption isotherm (i.e., the Brunauer-Emmett-Teller adsorptionisotherm). The specific surface area of the catalyst of the presentinvention is from 5 to 30 m²/g, preferably from 7 to 20 m²/g.

When the specific surface area of the catalyst is smaller than 5 m²/g,there cannot be obtained a satisfactory activity of the catalyst nor ahigh yield of the desired product. On the other hand, when the specificsurface area of the catalyst is larger than 30 m²/g, it is not sure thatany increase in the activity can be obtained, and rather it is probablethat the yield becomes poor and the activity is drastically degraded.Further, a problem arises in that, in the case of an ammoxidationreaction, the ammonium used as a raw material is wastefully burntwithout being used for producing the desired product.

With respect to the effect of the addition of a molybdenum compoundduring a catalytic oxidation or ammoxidation reaction for the purpose ofmaintaining the yield of the desired product, the present inventors haveunexpectedly found that the specific surface area of the catalyst usedhas a large influence on the effect of the addition. When the specificsurface area of the catalyst is smaller than 5 m²/g, almost no effectcan be achieved by the addition of a molybdenum compound. When thespecific surface area of the catalyst is larger than 30 m²/g, the effectof the addition of a molybdenum compound can be exhibited for a while,but the effect will be lost in a short time, that is, shortly thereoccurs a degradation of the catalyst, thus making it necessary toincrease the amount of the molybdenum compound added and increase thefrequency of addition of the molybdenum compound. The reason why thespecific surface area of the catalyst has such influence on the effectof the addition of a molybdenum compound has not yet been elucidated.However, it is presumed that, when the specific surface area of thecatalyst is smaller than 5 m²/g, the effective surface area of thecatalyst active species becomes too small, thus preventing the catalystactive species from fully receiving the effect of the addition of amolybdenum compound. It is also presumed that, when the specific surfacearea of the catalyst is larger than 30 m²/g, because the effectivesurface area of the catalyst active species is larger than 30 m²/g, theescaping of molybdenum from the catalyst active species is greatlyaccelerated disadvantageously.

In the present invention, the activity of the catalyst can berepresented by the activity as measured at a reaction temperature of440° C. Commercially, it is preferred that the activity is from 1.5 to10(×10³ hour⁻¹), more advantageously from 2 to 6(×10³ hour⁻¹), stillmore advantageously from 2 to 4(×10³ hour⁻¹). In the present invention,the activity of the catalyst is defined by the following formula:activity (hour⁻¹)=−3,600/(contact time)×ln((100−conversion of propane orisobutane)/100)

(wherein ln is natural logarithm)

Hereinbelow, explanations are made in detail on the process forproducing the catalyst of the present invention.

The catalyst of the present invention can be efficiently produced by,for example, a process for producing the catalyst of the presentinvention, which comprises the steps of:

providing an aqueous raw material mixture containing compounds ofmolybdenum, vanadium, niobium and antimony,

drying the aqueous raw material mixture to thereby obtain a driedcatalyst precursor, and

calcining the dried catalyst precursor under calcination conditionswherein the heating temperature of the dried catalyst precursor iscontinuously or intermittently elevated from a temperature which is lessthan 400° C. to a temperature which is in the range of from 550 to 700°C., wherein the calcination conditions are adjusted so that the catalystprecursor being calcined has a reduction ratio of from 8 to 12% when theheating temperature reaches 400° C., wherein the reduction ratio is asdefined in connection with the catalyst of the present invention,

thereby obtaining a catalyst having a reduction ratio of from 8 to 12%and a specific surface area of from 5 to 30 m²/g.

This process for producing the catalyst of the present invention isdescribed in detail. This process for producing the catalyst of thepresent invention comprises the following steps: a step for providing anaqueous raw material mixture, a step for drying the aqueous raw materialmixture to thereby obtain a dried catalyst precursor, and a step forcalcining the dried catalyst precursor. These steps are describedhereinbelow in detail.

<Aqueous Raw Material Mixture Preparation Step>

With respect to the molybdenum compound used as a source of molybdenumin the aqueous raw material mixture preparation step in the process ofthe present invention, there is no particular limitation. Preferredexamples of molybdenum compounds include ammonium heptamolybdate and thelike.

With respect to the vanadium compound as a source of vanadium, ammoniummetavanadate and the like can be advantageously used.

With respect to the niobium compound as a source of niobium, there canbe used at least one compound selected from the group consisting ofniobic acid, an inorganic acid salt of niobium, an organic acid salt ofniobium and the like. Of these, niobic acid is preferred.

Niobic acid is represented by the following formula: Nb₂O₅.nH₂O, whichis also known as “niobium hydroxide or “niobium oxide hydrate”.

As described in Unexamined Japanese Patent Application Laid-OpenSpecification No. Hei 11-47598, with respect to niobic acid, it ispreferred to use niobic acid in the form of a niobic acid-containingaqueous mixture which contains niobic acid, a dicarboxylic acid (e.g.oxalic acid) and ammonia, wherein the molar ratio (dicarboxylic acid/Nbmolar ratio) of the dicarboxylic acid to the niobic acid in terms ofniobium is in the range of from 1 to 4 and the molar ratio (ammonia/Nbmolar ratio) of the ammonia to the niobic acid in terms of niobium is 2or less.

With respect to the antimony compound as a source of antimony, antimonyoxide or the like can be advantageously used. Especially preferred isdiantimony trioxide.

In the case of producing a silica carrier-supported catalyst of thepresent invention, a silica sol or a fumed silica can be advantageouslyused as a source of silica.

In the present invention, water is generally used as an aqueous medium,but in order to adjust the solubility of the compounds in the aqueousmedium, if desired, there can be used water containing an alcohol in anamount within a range which does not cause any adverse effects on thecatalyst obtained. Examples of alcohols used in the present inventioninclude C₁-C₄ alcohols and the like.

Hereinbelow, a specific example of a method for preparing an aqueous rawmaterial mixture is explained, taking as an example the case which usesthe above-mentioned preferred raw material compounds as sources of thecomponent elements of the oxide catalyst of the present invention.

Ammonium heptamolybdate, ammonium metavanadate and diantimony trioxideare added to water, followed by heating of the resultant mixture tothereby obtain an aqueous mixture (A). It is preferred that the heatingis performed while stirring the mixture. It is preferred that theheating temperature is 50° C. or more, more advantageously in the rangeof from 50° C. to the boiling point, still more advantageously in therange of from 70° C. to the boiling point. It is further preferred thatthe heating temperature is in the range of from 80 to 100° C. Theheating may be performed under reflux by using a reflux equipment havinga condenser. Generally, in the case of heating under reflux, the boilingpoint is in the range of from about 101 to 102° C. The heating time ispreferably 0.5 hour or more. When the heating temperature is low (e.g.,lower than 50° C.), the heating time needs to be long. When the heatingtemperature is in the preferred range of from 80 to 100° C., the heatingtime is preferably in the range of from 1 to 5 hours.

It is preferred that, after the heating, hydrogen peroxide is added tothe aqueous mixture (A). By employing this operation, the molybdenum andthe vanadium which have been reduced during the preparation of theaqueous mixture (A) can be oxidized by the hydrogen peroxide added tothe aqueous mixture (A). When hydrogen peroxide is added to the aqueousmixture (A), it is preferred that the amount of the hydrogen peroxide issuch that the molar ratio (H₂O₂/Sb molar ratio) of the hydrogen peroxideto the antimony compound in terms of antimony is in the range of from0.01 to 20, more advantageously in the range of from 0.5 to 3, stillmore advantageously in the range of from 1 to 2.5. It is preferred that,after the addition of hydrogen peroxide, the aqueous mixture (A) isstirred at a temperature in the range of from 30 to 70° C. for 30minutes to 2 hours.

A niobium compound (e.g., niobic acid) is added to water, followed byheating of the resultant mixture to thereby obtain an aqueous liquid(B). It is preferred that the heating temperature is in the range offrom 50 to 100° C., more advantageously in the range of from 70 to 99°C., still more advantageously in the range of from 80 to 98° C. It ispreferred that the aqueous liquid (B) contains a dicarboxylic acid(e.g., oxalic acid) in addition to the niobium compound, wherein themolar ratio (dicarboxylic acid/Nb molar ratio) of the dicarboxylic acidto the niobium compound in terms of niobium is in the range of from 1 to4, more advantageously in the range of from 2 to 4. That is, in thiscase, niobic acid and oxalic acid are added to water, followed byheating and stirring of the resultant mixture to thereby obtain anaqueous liquid (B).

As a specific example of a method for preparing the above-mentionedaqueous liquid (B), there can be mentioned a method comprising thefollowing steps (1) to (3):

(1) mixing water, a dicarboxylic acid (e.g. oxalic acid) and a niobiumcompound (e.g. niobic acid) to thereby obtain a preliminaryniobium-containing aqueous solution or a niobium-containing aqueoussemisolution having suspended therein a part of the niobium compound;

(2) cooling the preliminary niobium-containing aqueous solution orniobium-containing aqueous semisolution to thereby precipitate a part ofthe dicarboxylic acid; and

(3) removing the precipitated dicarboxylic acid from the preliminaryniobium-containing aqueous solution, or removing the precipitateddicarboxylic acid and the suspended niobium compound from theniobium-containing aqueous semisolution,

thereby obtaining a niobium-containing aqueous liquid (B).

The aqueous liquid (B) obtained in the above method usually has adicarboxylic acid/Nb molar ratio within the range of from 2 to 4.

In step (1) of this method, it is especially preferred that oxalic acidis used as the dicarboxylic acid. With respect to the niobium compoundused in step (1) of this method, there can be mentioned niobic acid andniobium hydrogenoxalate. These niobium compounds can be used in the formof a solid or in the form of a dispersion in an appropriate medium.

When niobium hydrogenoxalate is used as the niobium compound, thedicarboxylic acid may not be used. When niobic acid is used as theniobium compound, in order to remove acidic impurities with which theniobic acid may have been contaminated during the production thereof,the niobic acid may be washed with an aqueous ammonia solution and/orwater prior to use.

It is preferred to use, as the niobium compound, a freshly preparedniobium compound. However, in the above-mentioned method, a niobiumcompound can be used which is slightly denatured (for example bydehydration) as a result of a long-term storage and the like.

In step (1) of this method, the dissolution of the niobium compound canbe promoted by the addition of a small amount of an aqueous ammonia orby heating.

The concentration of the niobium compound (in terms of niobium) in thepreliminary niobium-containing aqueous solution or aqueous semisolutionis preferably selected within the range of from 0.2 to 0.8 mol/kg of thesolution or semisolution. The dicarboxylic acid is preferably used in anamount such that the molar ratio of the dicarboxylic acid to the nobiumcompound in terms of niobium is approximately 3 to 6. When an excessamount of the dicarboxylic acid is used, a large amount of the niobiumcompound can be dissolved in the aqueous solution of dicarboxylic acid;however, a disadvantage is likely to arise in that the amount of thedicarboxylic acid which is caused to precipitate by cooling the obtainedpreliminary niobium-containing aqueous solution or semisolution becomestoo large, thus decreasing the utilization of the dicarboxylic acid. Onthe other hand, when an unsatisfactory amount of the dicarboxylic acidis used, a disadvantage is likely to arise in that a large amount of theniobium compound remains undissolved and is suspended in the aqueoussolution of the dicarboxylic acid to form a semisolution, wherein thesuspended niobium compound is removed from the semisolution, thusdecreasing the degree of utilization of the niobium compound.

The cooling operation in step (2) is not particularly limited. Thecooling can be performed simply, for example, by means of ice.

The removal of the precipitated dicarboxylic acid (or precipitateddicarboxylic acid and the dispersed niobium compound) in step (3) can beeasily performed by a conventional method, for example, by decantationor filtration.

When the dicarboxylic acid/Nb molar ratio of the obtainedniobium-containing aqueous solution is outside the range of from 2 to 4,either the niobium compound or dicarboxylic acid may be added to theaqueous liquid (B) so that the dicarboxylic acid/Nb molar ratio of thesolution falls within the above-mentioned range. However, in general,such an operation is unnecessary since an aqueous liquid (B) having thedicarboxylic acid/Nb molar ratio within the range of from 2 to 4 can beprepared by appropriately controlling the concentration of the niobiumcompound, the ratio of the dicarboxylic acid to the niobium compound andthe cooling temperature of the above-mentioned preliminaryniobium-containing aqueous solution or semisolution.

Thus, the aqueous liquid (B) can be prepared in the manner describedabove. However, the aqueous liquid (B) may also be prepared so as tocontain further component(s).

Specifically, it is preferred that at least a part of the aqueous liquid(B) containing a niobium compound or containing a mixture of a niobiumcompound and a dicarboxylic acid is used in the form of a mixturethereof with hydrogen peroxide. In this case, it is more preferred thatthe amount of the hydrogen peroxide is such that the molar ratio(H₂O₂/Nb molar ratio) of the hydrogen peroxide to the niobium compoundin terms of niobium is in the range of from 0.5 to 20, moreadvantageously in the range of from 1 to 20.

It is also preferred that at least a part of the aqueous liquid (B)containing a niobium compound or containing a mixture of a niobiumcompound and a dicarboxylic acid is used in the form of a mixturethereof with hydrogen peroxide and an antimony compound (e.g. diantimonytrioxide). In this case, it is more preferred that the amount of thehydrogen peroxide is such that the molar ratio (H₂O₂/Nb molar ratio) ofthe hydrogen peroxide to the niobium compound in terms of niobium is inthe range of from 0.5 to 20, more advantageously in the range of from 1to 20; and that the amount of the antimony compound mixed with the atleast a part of the aqueous liquid (B) and the hydrogen peroxide is suchthat the molar ratio (Sb/Nb molar ratio) of the antimony compound interms of antimony to the niobium compound in terms of niobium is notmore than 5, more advantageously in the range of from 0.01 to 2.

The aqueous mixture (A) and aqueous liquid (B) are mixed together in anappropriate ratio in accordance with the desired composition of thecatalyst, to thereby obtain an aqueous raw material mixture. Generally,the aqueous raw material mixture is obtained in the form of a slurry.The content of the aqueous medium in the aqueous raw material mixture isgenerally in the range of from 50 to less than 100% by weight,preferably in the range of from 70 to 95% by weight, more preferably inthe range of from 75 to 90% by weight.

In the case of producing a silica carrier-supported catalyst of thepresent invention, the aqueous raw material mixture is prepared so as tocontain a source of silica (namely, a silica sol or a fumed silica). Theamount of the source of silica can be appropriately adjusted inaccordance with the amount of the silica carrier in the catalyst to beobtained.

<Drying Step>

The above-obtained aqueous raw material mixture is dried to therebyobtain a dried catalyst precursor. The drying can be conducted byconventional methods, such as spray drying or evaporation drying. It ispreferred that a spray drying method is employed to thereby obtain afine, spherical dried catalyst precursor. The spray drying can beconducted by centrifugation, by the two-phase flow nozzle method or bythe high pressure nozzle method. As a heat source for drying, it ispreferred to use air which has been heated by steam, an electric heaterand the like. It is preferred that the temperature of the spray dryer atan entrance to the dryer section thereof is from 150 to 300° C., andthat the temperature of the spray dryer at an exit from the dryersection thereof is from 100 to 160° C.

<Calcination Step>

In the calcination step, the dried catalyst precursor obtained in thedrying step is calcined so as to obtain an oxide catalyst. Thecalcination can be conducted by using a rotary kiln, a fluidized-bedkiln or the like. When the calcination of the dried catalyst precursoris conducted in a stationary state, problems possibly arise in that thedried catalyst precursor cannot be evenly calcined, thus leading to adeterioration of the properties of the catalyst obtained and also to abreakage or cracking of the catalyst obtained.

The calcination is conducted so that the obtained oxide catalyst canhave a reduction ratio of from 8 to 12% and a specific surface are offrom 5 to 30 m²/g. Specifically, the calcination is conducted undercalcination conditions wherein the heating temperature of the driedcatalyst precursor is continuously or intermittently elevated from atemperature which is less than 400° C. to a temperature which is in therange of from 550 to 700° C., wherein the calcination conditions areadjusted so that the catalyst precursor being calcined has a reductionratio of from 8 to 12% when the heating temperature reaches 400° C.,thereby obtaining a catalyst having a reduction ratio of from 8 to 12%and a specific surface area of from 5 to 30 m²/g.

The calcination can be conducted in air or under a flow of air. However,at least a part of the calcination is preferably conducted in anatmosphere of an inert gas (e.g., under a flow of an inert gas), such asnitrogen gas which is substantially free of oxygen.

Especially, when the above-mentioned aqueous raw material mixturepreparation step contains an operation in which hydrogen peroxide isadded to the aqueous mixture (A), thereby oxidizing molybdenum andvanadium in the aqueous mixture (A) almost to their respective maximumoxidation numbers, it is preferred that the calcination of the obtaineddried catalyst precursor is conducted under a flow of an inert gas, suchas nitrogen gas which is substantially free of oxygen. The driedcatalyst precursor generally contains an ammonium radical, an organicacid, an inorganic acid and the like, as well as some water. When thecalcination is conducted under a flow of an inert gas which issubstantially free of oxygen, those compounds contained in the driedcatalyst precursor undergo evaporation, decomposition and the like,wherein these evaporation, decomposition and the like cause a reductionof the component elements in the catalyst precursor. When the componentelements in the dried catalyst precursor to be subjected to thecalcination respectively exhibit almost the maximum oxidation numbersthereof, the desired range of reduction ratio of the catalyst can beachieved simply by conducting the calcination so as to cause thecomponent elements to undergo a reduction during the calcination; thus,in this case, the calcination can be conducted in a simple, commerciallyadvantageous manner.

On the other hand, it is also possible to add an oxidant or a reductantto the atmosphere in which the calcination is performed, to therebyobtain the desired range of reduction ratio.

When the calcination is performed in a batchwise manner, the inert gasis supplied at a flow rate of not less than 50 N liters/hour/kg of thedried catalyst precursor, preferably in the range of from 50 to 5,000 Nliters/hour/kg of the dried catalyst precursor, more preferably in therange of from 50 to 3,000 N liters/hour/kg of the dried catalystprecursor (wherein N liter means liter as measured under the normaltemperature and pressure conditions, namely, at 0° C. under 1 atm.).

When the calcination is performed in a continuous manner, the inert gasis supplied at a flow rate of not less than 50 N liters/kg of the driedcatalyst precursor, preferably in the range of from 50 to 5,000 Nliters/kg of the dried catalyst precursor, more preferably in the rangeof from 50 to 3,000 N liters/kg of the dried catalyst precursor. In thecase of the calcination performed in a continuous manner, there is noparticular limitation with respect to the flow directions of the inertgas and the dried catalyst precursor, and the inert gas and the driedcatalyst precursor may be supplied either in a counter flow mode or in aparallel flow mode. However, preferred is a counter flow mode, becausegaseous substances are generated from the dried catalyst precursor, anda small amount of air comes into the calcination apparatus together withthe dried catalyst precursor.

The reduction ratio of the obtained catalyst is generally influenced bythe following factors: the amounts of organic substances contained inthe dried catalyst precursor, such as an oxalic acid; the amount ofammonium radical derived from an ammonium salt used as a raw material;the rate of the heating temperature elevation at the time of starting ofthe calcination; the amount of inert gas, in the case where thecalcination is conducted in an atmosphere of an inert gas; and thetemperature and time of the calcination, in the case where thecalcination is conducted in an atmosphere of air. For obtaining acatalyst which has a reduction ratio in the range of from 8 to 12%, itis important to calcine the dried catalyst precursor under calcinationconditions wherein the heating temperature of the dried catalystprecursor is elevated from a temperature which is less than 400° C., tothereby decompose the oxalate radical, the ammonium radical and the likecontained in the dried catalyst precursor, thereby substantiallycompleting the generation of gas from the catalyst precursor, so thatthe catalyst precursor being calcined has a reduction ratio of from 8 to12% when the heating temperature reaches 400° C.

On the other hand, the specific surface area of the obtained oxidecatalyst is influenced by the heating temperature and time of the finalcalcination (final heating), and the amount of the silica carrier in thecase of a catalyst comprising a silica carrier having supported thereonthe oxide. However, the specific surface area of the obtained oxidecatalyst is largely influenced, especially by the reduction ratio of thecatalyst precursor at the time when the heating temperature reaches 400°C., and by the final heating temperature of the calcination. The finalstage of the calcination is performed at a temperature in the range offrom 550 to 700° C. for a time of from 0.5 to 20 hours. The higher thefinal heating temperature and the longer the final heating time, thesmaller the specific surface area of the obtained catalyst. Also, thelower the reduction ratio of the catalyst precursor at the time when theheating temperature reaches 400° C., the smaller the specific surfacearea of the obtained catalyst; and, therefore, the higher the reductionratio of the catalyst precursor at the time when the heating temperaturereaches 400° C., the larger the specific surface area of the obtainedcatalyst. For obtaining a catalyst which has a specific surface area inthe range of from 5 to 30 m²/g, it is especially important to calcinethe dried catalyst precursor under calcination conditions wherein thecatalyst precursor being calcined has a reduction ratio of from 8 to 12%when the heating temperature reaches 400° C., and to perform the finalstage of calcination at a heating temperature in the range of from 550to 700° C.

The calcinacion can be performed in a single stage; however, for thepurpose of efficiently producing a catalyst which has a reduction ratioin the range of from 8 to 12% and a specific surface area in the rangeof from 5 to 30 m²/g, it is preferred that the calcination comprises apreliminary calcination and a final calcination, wherein the preliminarycalcination is performed at a temperature in the range of from 250 to400° C. and the final calcination is performed at a temperature in therange of from 550 to 700° C. The preliminary calcination and the finalcalcination may be performed either successively or completelyseparately. Further, each of the preliminary calcination and the finalcalcination may be performed in multiple stages.

For the measurement of the reduction ratio of the catalyst precursorbeing calcined, a specimen which is at the heating temperature may bequickly taken out of the calcination apparatus. However, since theheating temperature is high, it is possible that the catalyst precursortaken out is oxidized as it contacts air, thus causing the reductionratio of the catalyst precursor to be changed. Therefore, for preventinga change in the reduction ratio of the catalyst precursor, it is desiredthat the catalyst precursor being calcined is allowed to cool in situ toroom temperature before being taken out of the calcinating apparatus,and the catalyst precursor taken out in this manner is used as arepresentative specimen.

As specific examples of methods for causing the catalyst precursor beingcalcined to have a reduction ratio in the desired range when the heatingtemperature reaches 400° C., there can be mentioned the followingmethods: a method involving a control of the preliminary calcinationtemperature; a method involving an addition of an oxidant, such asoxygen, to an atmosphere in which the calcination is performed; a methodinvolving an addition of a reductant to an atmosphere in which thecalcination is performed; and a method of using the above-mentionedmethods in combination. Hereinbelow, each method is described in detail.

The above-mentioned method involving a control of the preliminarycalcination temperature is a method in which the temperature for thepreliminary calcination is controlled so as to cause the catalystprecursor being calcined to have a reduction ratio in the desired rangewhen the heating temperature reaches 400° C. Generally, the lower thepreliminary calcination temperature, the lower the reduction ratio ofthe catalyst precursor being calcined; and, the higher the preliminarycalcination temperature, the higher the reduction ratio of the catalystprecursor being calcined. This way, the reduction ratio of the catalystprecursor being calcined can be adjusted by controlling the preliminarycalcination temperature.

When the calcination of the dried catalyst precursor is effected, theheating temperature of the dried catalyst precursor is continuously orintermittently elevated from a temperature which is less than 400° C.,more preferably less than 250° C.

The preliminary calcination is preferably performed under a flow of aninert gas, at a heating temperature in the range of from 250 to 400° C.,more advantageously in the range of from 300 to 400° C. It is preferredthat the heating temperature is maintained at a constant level in therange of from 250 to 400° C.; however, the heating temperature mayfluctuate, or may be slowly elevated or lowered within the range of from250 to 400° C. It is preferred that the heating temperature ismaintained for 30 minutes or more, more advantageously from 3 to 12hours.

The elevation of the heating temperature before reaching the preliminarycalcination temperature may be performed either at a constant rate sothat the temperature elevation profile becomes a straight line, or at anon-constant rate so that the temperature elevation profile becomes aconvex or concave curve.

With respect to the average temperature elevation rate during theelevation of the heating temperature before reaching the preliminarycalcination temperature, there is no limitation; however, it isgenerally in the range of from about 0.1 to 15° C./min, preferably from0.5 to 5° C./min, more preferably from 1 to 2° C./min.

The above-mentioned “method involving an addition of an oxidant, such asoxygen, to an atmosphere in which the calcination is performed” so as tocause the catalyst precursor being calcined to have a reduction ratio inthe desired range when the heating temperature reaches 400° C., is amethod which can be employed for the purpose of lowering the reductionratio of the obtained catalyst. The term “calcination” means either orboth of the preliminary calcination and the final calcination. The“oxidant” which is added to an atmosphere in which the calcination isperformed means an oxidant which is contained in the inert gas suppliedto the calcination apparatus. The amount of the oxidant added isadjusted by controlling the oxidant concentration of the inert gassupplied to the calcination apparatus. The reduction ratio can becontrolled by adding an oxidant to the atmosphere in which thecalcination is performed. When oxygen is used as the oxidant, it ispreferred that air (or air-containing inert gas) is supplied to thecalcination apparatus, thereby utilizing the oxygen in the air as theoxidant.

The above-mentioned “method involving an addition of a reductant to anatmosphere in which the calcination is performed” so as to cause thecatalyst precursor being calcined to have a reduction ratio in thedesired range when the heating temperature reaches 400° C., is a methodwhich can be employed for the purpose of increasing the reduction ratioof the obtained catalyst. The term “calcination” means either or both ofthe preliminary calcination and the final calcination. The “reductant”which is added to an atmosphere in which the calcination is performedmeans a reductant which is contained in the inert gas supplied to thecalcinating apparatus. The amount of the reductant added is adjusted bycontrolling the reductant concentration of the inert gas supplied to thecalcinating apparatus. The reduction ratio can be controlled by adding areductant to the atmosphere in which the calcination is performed.Generally, ammonia can be used as the reductant.

In the case where the catalyst precursor being calcined does not have adesired value of reduction ratio when the heating temperature reaches400° C., a measure can be taken in which an oxidant or a reductant isadded to the atmosphere in which the calcination is performed, therebyadjusting the reduction ratio, wherein the necessary amount of theoxidant or the reductant is calculated from the difference between theactual reduction ratio and the desired reduction ratio.

The final calcination is preferably performed under a flow of an inertgas, at a heating temperature in the range of from 550 to 700° C., moreadvantageously in the range of from 580 to 650° C. It is preferred thatthe heating temperature is maintained at a constant level in the rangeof from 550 to 700° C.; however, the heating temperature may fluctuate,or may be slowly elevated or lowered within the range of from 550 to700° C. It is preferred that the final calcination is performed for aperiod of from 0.5 to 20 hours, more advantageously from 1 to 8 hours.During the final calcination, for the purpose of adjusting the reductionratio of the catalyst precursor being calcined, if desired, an oxidant(e.g. oxygen) or a reductant (e.g. ammonia) may be added to theatmosphere in the final calcination under a flow of an inert gas.

The elevation of the heating temperature after the preliminarycalcination and before reaching the final calcination temperature may beeither at a constant rate so that the temperature elevation profilebecomes a straight line, or at a non-constant rate so that thetemperature elevation profile becomes a convex or concave curve.

With respect to the average temperature elevation rate during theelevation of the heating temperature after the preliminary calcinationand before reaching the final calcination temperature, there is nolimitation; however, it is generally in the range of from about 0.1 to15° C./min, preferably from 0.5 to 10° C./min, more preferably from 1 to5° C./min.

With respect to the average temperature lowering rate during thelowering of the heating temperature after completion of the finalcalcination, it is generally in the range of from about 0.01 to 100°C./min, preferably from 0.05 to 100° C./min, more preferably from 0.1 to50° C./min, still more preferably from 0.5 to 10° C./min. It is alsopreferred that, during the temperature lowering, the heating temperatureis temporarily maintained at a temperature lower than the finalcalcination temperature, wherein the temporarily maintained temperatureis lower than the final calcination temperature by 5° C., moreadvantageously by 10° C., still more advantageously by 50° C. The timefor which the temperature is maintained is preferably 0.5 hour or more,more preferably 1 hour or more, still more preferably 3 hours or more,still further more preferably 10 hours or more.

In the determination of the reduction ratio, the value of (n₀−n) informula (2) defining the reduction ratio of the oxide catalyst can beobtained by a method in which the specimen is subjected to a redoxtitration using KMnO₄. With respect to both of the catalyst precursor(before completion of the calcination) and the catalyst (aftercompletion of the calcination), the value of (n₀−n) in formula (2) canbe obtained by a method in which the specimen is subjected to a redoxtitraion using KMnO₄. However, with respect to the conditions for theredox titration, there is a difference between the measuring operationin the case of the catalyst precursor (before completion of thecalcination) and the measuring operation in the case of the catalyst(after completion of the calcination). For each of the catalystprecursor and the catalyst, an example of a method for measuring thevalue of (n₀−n) in formula (2) is described below.

In the case of the catalyst precursor (before completion of thecalcination), the measurement is performed as follows. About 200 mg of aspecimen of the catalyst precursor is weighed and put in a beaker. Anexcess amount of an aqueous KMnO₄ solution having a predeterminedconcentration is added to the beaker. Then, 150 ml of purified water at70° C. and 2 ml of a 1:1 sulfuric acid (i.e., an aqueous sulfuric acidsolution obtained by mixing together a concentrated sulfuric acid andwater in a volume ratio of 1/1) are added to the beaker, and the mouthof the beaker is covered with a watch glass, and the beaker is placed ina hot water bath at 70° C.±2° C., and the contents of the beaker arestirred for 1 hour, thereby oxidizing the specimen. Then, it isconfirmed that the liquid in the beaker is purple since KMnO₄ was usedin an excess amount and therefore an unreacted KMnO₄ is present in theliquid in the beaker. After the oxidization, the resultant reactionmixture in the beaker is subjected to filtration using a filter paper,and all filtrate is recovered. An aqueous sodium oxalate solution havinga predetermined concentration is added to the recovered filtrate so thatsodium oxalate (Na₂C₂O₄) is used in an excess amount, relative to theKMnO₄ present in the filtrate. The resultant mixture is heated to 70° C.while stirring. It is confirmed that the mixture becomes colorless andtransparent. Then, 2 ml of a 1:1 sulfuric acid is added to the mixture,and the resultant mixture is stirred and kept at 70° C.±2° C. Withrespect to the mixture, a titration using an aqueous KMnO₄ solutionhaving a predetermined concentration is performed while stirring andkeeping the mixture at 70° C.±2° C. The dripping of the aqueous KMnO₄solution is ended when the mixture has a slight pink color which lastsfor about 30 seconds. From all amount of KMnO₄ used and all amount ofNa₂C₂O₄ used, the amount of KMnO₄ which was consumed for the oxidationof the specimen is determined. From this amount of KMnO₄, the value of(n₀−n) in formula (2) is calculated. Based on the thus obtained value of(n₀−n) in formula (2), the reduction ratio is obtained.

In the case of the catalyst (after completion of the calcination), themeasurement is performed as follows. A specimen of the catalyst isground in an agate mortar, and about 200 mg of the resultant groundspecimen is weighed and put in a beaker. 150 ml of purified water at 95°C. and 4 ml of a 1:1 sulfuric acid (i.e., an aqueous sulfuric acidsolution obtained by mixing together a concentrated sulfuric acid andwater in a volume ratio of 1/1) are added to the beaker, and theresultant mixture is stirred and kept at 95° C.±2° C. With respect tothe mixture, a titration using an aqueous KMnO₄ solution having apredetermined concentration is performed while stirring and keeping themixture at 95° C.±2° C. In this instance, when the aqueous KMnO₄solution is dripped into the mixture, the mixture temporarily has apurple color. The dripping of the aqueous KMnO₄ solution is slowlycontinued while confirming that the purple color does not last for 30seconds or more. The amount of the mixture in the beaker graduallydecreases in accordance with the evaporation of the water. Purifiedwater at 95° C. is added to the mixture in the beaker so as to maintainthe amount of the mixture at a constant level. The dripping of theaqueous KMnO₄ solution is finished when the mixture has a slight pinkcolor which lasts for about 30 seconds. Thus, the amount of KMnO₄ whichwas consumed for the oxidation of the specimen is determined. From thisamount of KMnO₄, the value of (n₀−n) in formula (2) is calculated. Basedon the thus obtained value of (n₀−n) in formula (2), the reduction ratiois obtained.

In addition to the above-described method for determining the value of(n₀−n) in formula (2), there can also be mentioned the below-describedmethod, which can be used for both of the case of the catalyst precursor(before completion of the calcination) and the case of the catalyst(after completion of the calcination). The specimen (the catalystprecursor or the catalyst) is heated to a temperature which is higherthan the highest temperature at which the specimen was heated during thecalcination, thereby completely oxidizing the specimen with oxygen,wherein the heating is performed under conditions which can preventvolatilization and escaping of the component elements. By the heating,the weight of the specimen is increased due to the binding of oxygen.After the heating, the weight difference between the specimen after theheating and the specimen before the heating is measured. From the weightdifference (the weight of the oxygen bound), the value of (n₀−n) informula (2) is calculated. Based on the thus obtained value of (n₀−n) informula (2), the reduction ratio is obtained.

The excellent catalyst of the present invention can be produced by thesimple method as described herein above. The thus obtained catalyst ofthe present invention can be used for producing an unsaturatedcarboxylic acid, i.e., for producing acrylic acid or methacrylic acid bya process which comprises reacting propane or isobutane with molecularoxygen in the gaseous phase in the presence of the catalyst of thepresent invention. The catalyst of the present invention can also beused for producing an unsaturated nitrile, i.e., for producingacrylonitrile or methacrylonitrile by a process which comprises reactingpropane or isobutane with ammonia and molecular oxygen in the gaseousphase in the presence of the catalyst of the present invention.

Propane, isobutane and ammonia used in the present invention need not beof a very high purity but may be of a commercial grade.

Examples of sources of molecular oxygen include air, pure oxygen andoxygen-rich air. Further, such a source of molecular oxygen may bediluted with helium, neon, argon, carbon dioxide, steam, nitrogen or thelike.

In the ammoxidation reaction, the molar ratio of ammonia to propane orisobutane used for the ammoxidation is generally in the range of from0.3 to 1.5, preferably from 0.8 to 1.2.

In each of the oxidation reaction and the ammoxidation reaction, themolar ratio of molecular oxygen to propane or isobutane used for theoxidation or ammoxidation is generally in the range of from 0.1 to 6,preferably from 0.1 to 4.

In each of the oxidation reaction and the ammoxidation reaction, thereaction pressure is generally in the range of from 0.5 to 5 atm,preferably from 1 to 3 atm.

In each of the oxidation reaction and the ammoxidation reaction, thereaction temperature is generally in the range of from 350 to 500° C.,preferably from 380 to 470° C.

In each of the oxidation reaction and the ammoxidation reaction, thetime of contact (contact time) between the gaseous feedstock mixture andthe catalyst is generally in the range of from 0.1 to 10 (sec·g/cc),preferably from 0.5 to 5 (sec·g/cc).

In the present invention, the contact time is defined by the followingformula:contact time (sec·g/cc)=(W/F)×273/(273+T)×P

wherein:

-   -   W represents the weight (g) of the catalyst,    -   F represents the flow rate (Ncc/sec) of the gaseous feedstock        mixture in the standard state (0° C., 1 atm),    -   T represents the reaction temperature (0° C.), and    -   P represents the reaction pressure (atm).

Each of the oxidation reaction and the ammoxidation reaction can beconducted in a conventional reactor, such as a fixed bed reactor, afluidized-bed reactor or a moving bed reactor. However, most preferredis a fluidized-bed reactor, because the use of a fluidized-bed reactoris advantageous in that the heat removal during the reaction can beeasily performed, and therefore the temperature of the catalyst bed canbe kept almost even, and that it is possible to take out the catalystfrom the reactor and to feed an additional amount of catalyst to thereactor while operating the reactor.

For conducting the reaction on a commercial scale stably for a longtime, it is preferred that a molybdenum compound is added to thereaction system. With respect to the molybdenum compound, there is noparticular limitation, and any molybdenum compound can be used as longas the compound contains molybdenum element. However, from the viewpointof easy handling and economy, it is preferred to use the same molybdenumcompound as used in the production of the catalyst, for example,ammonium heptamolybdate. The catalyst of the present invention isadvantageous in that, when a molybdenum compound is added to thecatalytic oxidation or ammoxidation reaction system as conventionallypracticed in the art for the purpose of maintaining a high yield bypreventing a catalyst degradation caused by the volatilization orescaping of molybdenum from the catalyst, the amount of molybdenumcompound added and the frequency of addition of molybdenum compound canbe decreased, as compared to the case of the use of conventionalcatalysts, so that the reaction can be performed economically.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, the present invention will be described in more detail withreference to the following Examples and Comparative Examples, whichshould not be construed as limiting the scope of the present invention.

In the following Examples and Comparative Examples, the results of theammoxidation were evaluated in terms of the conversion (%) of propane,the selectivity (%) for acrylonitrile, the yield (%) of acrylonitrile,and the catalyst activity, which are, respectively, defined as follows:

${{Conversion}\mspace{11mu}(\%)\mspace{11mu}{of}\mspace{14mu}{propane}} = {\frac{{mole}\mspace{14mu}{of}\mspace{14mu}{propane}\mspace{14mu}{reacted}}{{mole}\mspace{14mu}{of}\mspace{14mu}{propane}\mspace{14mu}{fed}} \times 100}$$\begin{matrix}{{Selectivity}\mspace{11mu}(\%)} \\{{for}\mspace{14mu}{acrylonitrile}}\end{matrix} = {\frac{{mole}\mspace{14mu}{of}\mspace{14mu}{acrylonitrile}\mspace{14mu}{formed}}{{mole}\mspace{14mu}{of}\mspace{14mu}{propane}\mspace{14mu}{reacted}} \times 100}$${{Yield}\mspace{11mu}(\%)\mspace{11mu}{for}\mspace{14mu}{acrylonitrile}} = \frac{{mole}\mspace{14mu}{of}\mspace{14mu}{acrylonitrile}\mspace{14mu}{formed}}{{mole}\mspace{14mu}{of}\mspace{14mu}{propane}\mspace{14mu}{fed}}$Activity (hour⁻¹)=−3,600/(contact time)×ln((100−conversion ofpropane)/100)

(wherein ln is natural logarithm)

The Method for Measuring the Reduction Ratio:

In the case of the catalyst precursor (being calcined) at the time whenthe heating temperature reached 400° C., the measurement of thereduction ratio was performed as follows.

About 200 mg of a specimen of the catalyst precursor was weighed and putin a beaker. An excess amount of an aqueous KMnO₄ solution having apredetermined concentration was added to the beaker. Then, 150 ml ofpurified water at 70° C. and 2 ml of a 1:1 sulfuric acid (i.e., anaqueous sulfuric acid solution obtained by mixing together aconcentrated sulfuric acid and water in a volume ratio of 1/1) wereadded to the beaker, and the mouth of the beaker was covered with awatch glass, and the beaker was placed in a hot water bath at 70° C.±2°C., and the contents of the beaker were stirred for 1 hour, therebyoxidizing the specimen. Then, it was confirmed that the liquid in thebeaker was purple since KMnO₄ had been used in an excess amount andtherefore an unreacted KMnO₄ was present in the liquid in the beaker.After the oxidization, the resultant reaction mixture in the beaker wassubjected to filtration using a filter paper, and all filtrate wasrecovered. An aqueous sodium oxalate solution having a predeterminedconcentration was added to the recovered filtrate so that sodium oxalate(Na₂C₂O₄) was used in an excess amount, relative to the KMnO₄ present inthe filtrate. The resultant mixture was heated to 70° C. while stirring.It was confirmed that the mixture became colorless and transparent.Then, 2 ml of a 1:1 sulfuric acid was added to the mixture, and theresultant mixture was stirred and kept at 70° C.±2° C. With respect tothe mixture, a titration using an aqueous KMnO₄ solution having apredetermined concentration was performed while stirring and keeping themixture at 70° C.±2° C. The dripping of the aqueous KMnO₄ solution wasfinished when the mixture had a slight pink color which lasted for about30 seconds. From all amount of KMnO₄ used and all amount of Na₂C₂O₄used, the amount of KMnO₄ which had been consumed for the oxidation ofthe specimen was determined. From this amount of KMnO₄, the value of(n₀−n) in formula (2) was calculated. Based on the thus obtained valueof (n₀−n) in formula (2), the reduction ratio was obtained.

In the case of the catalyst (after completion of the calcination), themeasurement of the reduction ratio was performed as follows.

A specimen of the catalyst was ground in an agate mortar, and about 200mg of the resultant ground specimen was weighed and put in a beaker. 150ml of purified water at 95° C. and 4 ml of a 1:1 sulfuric acid (i.e., anaqueous sulfuric acid solution obtained by mixing together aconcentrated sulfuric acid and water in a volume ratio of 1/1) wereadded to the beaker, and the resultant mixture was stirred and kept at95° C.±2° C. With respect to the mixture, a titration using an aqueousKMnO₄ solution having a predetermined concentration was performed whilestirring and keeping the mixture at 95° C.±2° C. In this instance, whenthe aqueous KMnO₄ solution was dripped into the mixture, the mixturetemporarily had a purple color. The dripping of the aqueous KMnO₄solution was slowly continued while confirming that the purple color didnot last for 30 seconds or more. The amount of the mixture in the beakergradually decreased in accordance with the evaporation of the water.Purified water at 95° C. was added to the mixture in the beaker so as tomaintain the amount of the mixture at a constant level. The dripping ofthe aqueous KMnO₄ solution was finished when the mixture had a slightpink color which lasted for about 30 seconds. Thus, the amount of KMnO₄which had been consumed for the oxidation of the specimen wasdetermined. From this amount of KMnO₄, the value of (n₀−n) in formula(2) was calculated. Based on the thus obtained value of (n₀−n) informula (2), the reduction ratio was obtained.

The Method for Measuring the Specific Surface Area:

The specific surface area of the catalyst was measured by the BETmethod, using a surface area analyzer “GEMINI 2360” (manufactured byMicromeritics, U.S.A., imported and sold by Shimadzu Corporation,Japan).

<Preparation of a Niobium-containing Aqueous Solution>

In accordance with the method as disclosed in Unexamined Japanese PatentApplication Laid-Open Specification No. Hei 11-253801, aniobium-containing aqueous solution was prepared as follows. To 8,450 gof water were added 1,290 g of niobic acid (Nb₂O₅ content: 80.2% byweight) and 4,905 g of oxalic acid dehydrate (H₂C₂O₄.2H₂O). The oxalicacid/niobium molar ratio in the resultant aqueous mixture was 5.0, andthe niobium concentration of the resultant aqueous mixture was 0.532mol/kg of the aqueous solution.

The obtained mixture was stirred at 95° C. for 1 hour, to thereby obtaina preliminary niobium-containing aqueous solution. The obtainedpreliminary niobium-containing aqueous solution was allowed to standstill while cooling with ice, to thereby precipitate solids. Theprecipitated solids in the solution were removed from the aqueoussolution by suction filtration, to thereby obtain a homogenized niobiumcompound-containing aqueous solution.

The same procedure as described hereinabove was repeated several times,and the resultant niobium compound-containing aqueous solutions weremixed together, and the resultant mixture was designated the“niobium-containing aqueous solution”. The oxalic acid/niobium molarratio in the niobium-containing aqueous solution was 2.40, as determinedby the following analysis method.

A 10 g sample solution was accurately taken from the niobium-containingaqueous solution and charged into a crucible. The sample solution wasdried overnight at 95° C., followed by calcination at 600° C. for 1hour, thereby obtaining 0.8639 g of Nb₂O₅. As a result, it was foundthat the niobium concentration of the niobium-containing aqueoussolution was 0.65 mol/kg of the aqueous solution.

Next, the oxalic acid concentration of the niobium-containing aqueoussolution was determined as follows. To a 300 ml glass beaker was added 3g of sample solution accurately taken from the niobium-containingaqueous solution, followed by addition of 200 ml of water having atemperature of about 80° C. and 10 ml of aqueous sulfuric acid solution(volume ratio of concentrated sulfuric acid to water=1/1), to therebyobtain a test solution. The obtained test solution was subjected totitration using 1/4 N KMnO₄ solution, while stirring the test solutionat 70° C. by using a hot stirrer. That is, the titration was conductedin accordance with the following reaction equation:2KMnO₄+3H₂SO₄+5H₂C₂O₄→K₂SO₄+2MnSO₄+10CO₂+8H₂O.An occurrence of a change in the color of the test solution inaccordance with the progress of titration was examined. That is, thepoint at which the test solution was caused to assume a very light pinkcolor due to the KMnO₄ and from which the test solution continued tohave the very light pink color for 30 seconds or more, was defined as anend point of the titration. From the amount of 1/4 N KMnO₄ solutionconsumed, the oxalic acid concentration of the niobium-containingaqueous solution was calculated using the above reaction formula. As aresult, it was found that the oxalic acid concentration of theniobium-containing aqueous solution was 1.56 mol/kg of the aqueoussolution.

The thus obtained niobium-containing aqueous solution was used as theniobium-containing aqueous solution (B₀) in the following processes forproducing catalysts.

EXAMPLE 1

An oxide catalyst represented by the formula:Mo₁V_(0.21)Nb_(0.09)Sb_(0.24)O_(n)/SiO₂(45% by weight) was prepared asfollows.

(Preparation of an Aqueous Raw Material Mixture)

To 4,640 g of water were added 931.4 g of ammonium heptamolybdate[(NH₄)₆Mo₇O₂₄.4H₂O], 128.8 g of ammonium metavanadate (NH₄VO₃) and 153.1g of diantimony trioxide (Sb₂O₃), and the resultant mixture was heatedwhile stirring at 90° C. for 2.5 hours, thereby obtaining an aqueousmixture A-1.

On the other hand, to 725.3 g of the niobium-containing aqueous solution(B₀) was added 154.4 g of 30% by weight aqueous hydrogen peroxide(H₂O₂). To the resultant mixture was further added 30.6 g of diantimonytrioxide (Sb₂O₃) slowly while keeping the temperature at about 20° C.,followed by stirring, to thereby obtain an aqueous liquid B-1.

Subsequently, the above-obtained aqueous mixture A-1 was cooled to 70°C., followed by addition of 1,960 g of a silica sol having an SiO₂content of 30.6% by weight. Then, to the resultant mixture was furtheradded 178.2 g of 30% by weight aqueous hydrogen peroxide (H₂O₂), and theresultant mixture was stirred at 50° C. for 1 hour. To the resultantmixture was further added the aqueous liquid B-1 to obtain a mixture. Tothe obtained mixture was further added a liquid obtained by dispersing300 g of a fumed silica having an average primary particle diameter ofabout 12 nm into 4,500 g of water, to thereby obtain an aqueous rawmaterial mixture.

(Preparation of a Dried Catalyst Precursor)

The thus obtained aqueous raw material mixture was subjected to spraydrying using a centrifugation type spray-drying apparatus, to therebyobtain a dried, microspherical particulate catalyst precursor. The inletand outlet temperatures of the dryer section of the apparatus were 210°C. and 120° C., respectively.

(Calcination)

480 g of the obtained dried catalyst precursor was charged into a SUScalcination tube (inner diameter: 3 inch), and then calcination wasperformed under a stream of nitrogen gas at a flow rate of 1.5 Nliters/min while rotating the calcination tube, under conditions whereinthe heating temperature was elevated to 345° C. over about 4 hours andthen maintained at 345° C. for 4 hours, and then, the heatingtemperature was elevated to 640° C. over 2 hours and then maintained at640° C. for 2 hours, to thereby obtain an oxide catalyst. During thecalcination, when the heating temperature reached 400° C., a part(specimen) of the catalyst precursor being calcined was taken out of thecalcination tube so as not to cause a reduction of the specimen, and thereduction ratio of the specimen of catalyst precursor was measured. Itwas found that the catalyst precursor had a reduction ratio of 10.3%.

The measurement of the reduction ratio was also performed with respectto the catalyst (after completion of the calcination). It was found thatthe obtained catalyst had a reduction ratio of 10.3%.

The specific surface area of the obtained catalyst was measured by theBET method using the above-mentioned surface area analyzer “GEMINI2360”, and it was found that the catalyst had a surface area of 16 m²/g.

EXAMPLE 2

An oxide catalyst represented by the formula:Mo₁V_(0.21)Nb_(0.09)Sb_(0.24)O_(n)/SiO₂(45% by weight) was prepared asfollows.

(Preparation of an Aqueous Raw Material Mixture)

To 4,640 g of water were added 931.4 g of ammonium heptamolybdate[(NH₄)₆Mo₇O₂₄.4H₂O], 128.8 g of ammonium metavanadate (NH₄VO₃) and 183.8g of diantimony trioxide (Sb₂O₃), and the resultant mixture was heatedwhile stirring at 90° C. for 2.5 hours, thereby obtaining an aqueousmixture A-2.

On the other hand, to 725.3 g of the niobium-containing aqueous solution(B₀) was added 106.9 g of 30% by weight aqueous hydrogen peroxide(H₂O₂), followed by stirring, to thereby obtain an aqueous liquid B-2.

Subsequently, the above-obtained aqueous mixture A-2 was cooled to 70°C., followed by addition of 1,960 g of a silica sol having an SiO₂content of 30.6% by weight. Then, to the resultant mixture was furtheradded 213.8 g of 30% by weight aqueous hydrogen peroxide (H₂O₂), and theresultant mixture was stirred at 50° C. for 1 hour. To the resultantmixture was further added the aqueous liquid B-2 to obtain a mixture. Tothe obtained mixture was further added a liquid obtained by dispersing300 g of a fumed silica having an average primary particle diameter ofabout 12 nm into 4,500 g of water, to thereby obtain an aqueous rawmaterial mixture.

(Preparation of a Dried Catalyst Precursor)

The thus obtained aqueous raw material mixture was subjected to spraydrying using a centrifugation type spray-drying apparatus, to therebyobtain a dried, microspherical particulate catalyst precursor. The inletand outlet temperatures of the dryer section of the apparatus were 210°C. and 120° C., respectively.

The above-described procedure for producing an aqueous raw materialmixture and producing a dried catalyst precursor was repeated fivetimes, and the resultant dried catalyst precursors were mixed together.The thus obtained dried catalyst precursor was then subjected tocalcination as described below.

(Calcination)

The obtained dried catalyst precursor was fed at a feeding rate of 80g/hour into a SUS continuous calcination apparatus having a calcinationtube (inner diameter: 3 inch; length: 89 cm), and then calcination wasperformed under a stream of nitrogen gas fed in a counter flow (i.e., ina flow direction opposing the feeding direction of the dried catalystprecursor) at a flow rate of 1.5 N liters/min while rotating thecalcination tube, under conditions wherein the heating temperature waselevated to 345° C. over about 4 hours and then maintained at 345° C.for 4 hours. The resultant preliminarily calcined catalyst precursor wascollected at the outlet of the calcination tube. A part (specimen) ofthe preliminarily calcined catalyst precursor was taken and heated in anitrogen atmosphere to 400° C., and then the reduction ratio of thespecimen of preliminarily calcined catalyst precursor was measured. Itwas found that the preliminarily calcined catalyst precursor had areduction ratio of 10.4%.

The obtained preliminarily calcined catalyst precursor was fed at afeeding rate of 130 g/hour into a SUS continuous calcination apparatushaving a calcination tube (inner diameter: 3 inch; length: 89 cm), andthen calcination was performed under a stream of nitrogen gas at a flowrate of 1.5 N liters/min while rotating the calcination tube, underconditions wherein the heating temperature was elevated to 640° C. overabout 4 hours and then maintained at 640° C. for 2 hours, to therebyobtain a catalyst. The obtained catalyst (after completion of the finalcalcination) was collected at the outlet of the calcination tube. Thereduction ratio and specific surface area of the catalyst were measured.It was found that the catalyst had a reduction ratio of 10.4% and aspecific surface area of 17 m²/g.

EXAMPLE 3

Using the dried catalyst precursor obtained in Example 2, thecalcination was performed in substantially the same manner as in Example1, except that the heating temperatures for the preliminary calcinationand the final calcination were 350° C. and 620° C., respectively. Duringthe calcination, when the heating temperature reached 400° C., a part(specimen) of the catalyst precursor being calcined was taken out of thecalcination tube so as not to cause a reduction of the specimen, and thereduction ratio of the specimen of catalyst precursor was measured. Itwas found that the catalyst precursor had a reduction ratio of 10.8%.The reduction ratio and specific surface area of the catalyst (aftercompletion of the calcination) were measured. It was found that thecatalyst had a reduction ratio of 10.8% and a specific surface area of19 m²/g.

COMPARATIVE EXAMPLE 1

An oxide catalyst represented by the formula:Mo₁V_(0.21)Nb_(0.09)Sb_(0.24)O_(n)/SiO₂(45% by weight) was prepared asfollows.

(Preparation of an Aqueous Raw Material Mixture)

To 4,640 g of water were added 931.4 g of ammonium heptamolybdate[(NH₄)₆Mo₇O₂₄.4H₂O], 128.8 g of ammonium metavanadate (NH₄VO₃) and 183.8g of diantimony trioxide (Sb₂O₃), and the resultant mixture was heatedwhile stirring at 90° C. for 2.5 hours, thereby obtaining an aqueousmixture A-3.

On the other hand, 725.3 g of the niobium-containing aqueous solution(Bo) was taken and designated an aqueous liquid B-3.

Subsequently, the above-obtained aqueous mixture A-3 was cooled to 70°C., followed by addition of 1,960 g of a silica sol having an SiO₂content of 30.6% by weight. Then, to the resultant mixture was furtheradded the aqueous liquid B-3 to obtain a mixture. To the obtainedmixture was further added a liquid obtained by dispersing 300 g of afumed silica having an average primary particle diameter of about 12 nminto 4,500 g of water, to thereby obtain an aqueous raw materialmixture.

(Preparation of a Dried Catalyst Precursor)

The thus obtained aqueous raw material mixture was subjected to spraydrying using a centrifugation type spray-drying apparatus, to therebyobtain a dried, microspherical particulate catalyst precursor. The inletand outlet temperatures of the dryer section of the apparatus were 210°C. and 120° C., respectively.

(Calcination)

480 g of the obtained dried catalyst precursor was charged into a SUScalcination tube (inner diameter: 3 inch), and then calcination wasperformed under a stream of nitrogen gas at a flow rate of 1.5 Nliters/min while rotating the calcination tube, under conditions whereinthe heating temperature was elevated to 345° C. over about 4 hours andthen maintained at 345° C. for 4 hours, and then, the heatingtemperature was elevated to 660° C. over 2 hours and then maintained at660° C. for 2 hours, to thereby obtain an oxide catalyst. During thecalcination, when the heating temperature reached 400° C., a part(specimen) of the catalyst precursor being calcined was taken out of thecalcination tube so as not to cause a reduction of the specimen, and thereduction ratio of the specimen of catalyst precursor was measured. Itwas found that the catalyst precursor had a reduction ratio of 15.4%.The reduction ratio and specific surface area of the catalyst (aftercompletion of the calcination) were measured. It was found that thecatalyst had a reduction ratio of 15.5% and a specific surface area of25 m²/g.

COMPARATIVE EXAMPLE 2

An oxide catalyst represented by the formula:Mo₁V_(0.21)Nb_(0.09)Sb_(0.24)O_(n)/SiO₂(45% by weight) was prepared asfollows.

(Preparation of an Aqueous Raw Material Mixture and a Dried CatalystPrecursor)

A dried catalyst precursor was prepared in substantially the same manneras in Comparative Example 1.

(Calcination)

480 g of the obtained dried catalyst precursor was charged into a SUScalcination tube (inner diameter: 3 inch), and then calcination wasperformed under a stream of air at a flow rate of 1.5 N liters/min whilerotating the calcination tube, under conditions wherein the heatingtemperature was elevated to 400° C. over about 4 hours and thenmaintained at 400° C. for 4 hours, and then, the heating temperature waselevated to 640° C. over 2 hours and then maintained at 640° C. for 2hours, to thereby obtain an oxide catalyst. During the calcination, whenthe heating temperature reached 400° C., a part (specimen) of thecatalyst precursor being calcined was taken out of the calcination tubeso as not to cause a reduction of the specimen, and the reduction ratioof the specimen of catalyst precursor was measured. It was found thatthe catalyst precursor had a reduction ratio of 1.1%. The reductionratio and specific surface area of the catalyst (after completion of thecalcination) were measured. It was found that the catalyst had areduction ratio of 1.0% and a specific surface area of 11 m²/g.

COMPARATIVE EXAMPLE 3

Using the dried catalyst precursor obtained in Example 2, thecalcination was performed in substantially the same manner as in Example1, except that the heating temperatures for the preliminary calcinationand the final calcination were 460° C. and 640° C., respectively. Duringthe calcination, when the heating temperature reached 400° C., a part(specimen) of the catalyst precursor being calcined was taken out of thecalcination tube so as not to cause a reduction of the specimen, and thereduction ratio of the specimen of catalyst precursor was measured. Itwas found that the catalyst precursor had a reduction ratio of 13.2%.The reduction ratio and specific surface area of the catalyst (aftercompletion of the calcination) were measured. It was found that thecatalyst had a reduction ratio of 13.2% and a specific surface area of21 m²/g.

COMPARATIVE EXAMPLE 4

Using the dried catalyst precursor obtained in Example 2, thecalcination was performed in substantially the same manner as in Example1, except that the heating temperatures for the preliminary calcinationand the final calcination were 350° C. and 500° C., respectively. Duringthe calcination, when the heating temperature reached 400° C., a part(specimen) of the catalyst precursor being calcined was taken out of thecalcination tube so as not to cause a reduction of the specimen, and thereduction ratio of the specimen of catalyst precursor was measured. Itwas found that the catalyst precursor had a reduction ratio of 10.8%.The reduction ratio and specific surface area of the catalyst (aftercompletion of the calcination) were measured. It was found that thecatalyst had a reduction ratio of 10.8% and a specific surface area of45 m²/g.

COMPARATIVE EXAMPLE 5

Using the dried catalyst precursor obtained in Example 2, thecalcination was performed in substantially the same manner as in Example1, except that the heating temperatures for the preliminary calcinationand the final calcination were 350° C. and 800° C., respectively. Duringthe calcination, when the heating temperature reached 400° C., a part(specimen) of the catalyst precursor being calcined was taken out of thecalcination tube so as not to cause a reduction of the specimen, and thereduction ratio of the specimen of catalyst precursor was measured. Itwas found that the catalyst precursor had a reduction ratio of 10.7%.The reduction ratio and specific surface area of the catalyst (aftercompletion of the calcination) were measured. It was found that thecatalyst had a reduction ratio of 10.8% and a specific surface area of 4m²/g.

EXAMPLE 4

Using the dried catalyst precursor obtained in Example 2, thecalcination was performed in substantially the same manner as in Example1, except that the calcination was performed under a stream of nitrogengas containing 400 ppm oxygen, and that the heating temperatures for thepreliminary calcination and the final calcination were 460° C. and 640°C., respectively. During the calcination, when the heating temperaturereached 400° C., a part (specimen) of the catalyst precursor beingcalcined was taken out of the calcination tube so as not to cause areduction of the specimen, and the reduction ratio of the specimen ofcatalyst precursor was measured. It was found that the catalystprecursor had a reduction ratio of 11.0%. The reduction ratio andspecific surface area of the catalyst (after completion of thecalcination) were measured. It was found that the catalyst had areduction ratio of 11.1% and a specific surface area of 18 m²/g.

EXAMPLE 5

(Evaluation of the Catalyst Activity)

2.0 g of the oxide catalyst obtained in Example 1 was charged into afixed-bed type reaction tube having an inner diameter of 10 mm. Agaseous feedstock mixture having a molar ratio ofpropane:ammonia:oxygen:helium of 1:1.2:2.8:11 was fed into the reactiontube. The reaction temperature was 440° C., and the reaction pressurewas the normal pressure, namely, under 1 atm. The contact time betweenthe oxide catalyst and the gaseous mixture of the feedstocks was 2.8(sec·g/cc). The results are shown in Table 1.

(Evaluation of the Yield of Acrylonitrile)

45 g of the oxide catalyst obtained in Example 1 was charged into aVycor glass fluidized-bed type reaction tube having an inner diameter of25 mm. A gaseous feedstock mixture having a molar ratio ofpropane:ammonia:oxygen:helium of 1:1:3.2:12 was fed into the reactiontube. The reaction temperature was 440° C., the reaction pressure wasthe normal pressure, namely, under 1 atm, and the contact time was 3.2(sec·g/cc).

0.1 g of ammonium heptamolybdate was added to the reaction system, 1,600hours after the start of the reaction. The results are shown in Table 2.

EXAMPLE 6

(Evaluation of the Catalyst Activity)

Using the oxide catalyst obtained in Example 2, the ammoxidation ofpropane was performed for evaluating the catalyst activity, insubstantially the same manner as in Example 5. The results are shown inTable 1.

(Evaluation of the Yield of Acrylonitrile)

Using the oxide catalyst obtained in Example 2, the ammoxidation ofpropane was performed for evaluating the yield of acrylonitrile, insubstantially the same manner as in Example 5. The results are shown inTable 2.

EXAMPLE 7

(Evaluation of the Catalyst Activity)

Using the oxide catalyst obtained in Example 3, the ammoxidation ofpropane was performed for evaluating the catalyst activity, insubstantially the same manner as in Example 5. The results are shown inTable 1.

(Evaluation of the Yield of Acrylonitrile)

Using the oxide catalyst obtained in Example 3, the ammoxidation ofpropane was performed for evaluating the yield of acrylonitrile, insubstantially the same manner as in Example 5. The results are shown inTable 2.

COMPARATIVE EXAMPLE 6

(Evaluation of the Catalyst Activity)

Using the oxide catalyst obtained in Comparative Example 1, theammoxidation of propane was performed for evaluating the catalystactivity, in substantially the same manner as in Example 5. The resultsare shown in Table 1.

(Evaluation of the Yield of Acrylonitrile)

Using the oxide catalyst obtained in Comparative Example 1, theammoxidation of propane was performed for evaluating the yield ofacrylonitrile, in substantially the same manner as in Example 5, exceptthat the reaction was terminated 24 hours after the start of thereaction, because of too low an yield obtained. The results are shown inTable 2.

COMPARATIVE EXAMPLE 7

(Evaluation of the Catalyst Activity)

Using the oxide catalyst obtained in Comparative Example 2, theammoxidation of propane was performed for evaluating the catalystactivity, in substantially the same manner as in Example 5. The resultsare shown in Table 1.

(Evaluation of the Yield of Acrylonitrile)

Using the oxide catalyst obtained in Comparative Example 2, theammoxidation of propane was performed for evaluating the yield ofacrylonitrile, in substantially the same manner as in Example 5, exceptthat the reaction was terminated 24 hours after the start of thereaction, because of too low an yield obtained. The results are shown inTable 2.

COMPARATIVE EXAMPLE 8

(Evaluation of the Catalyst Activity)

Using the oxide catalyst obtained in Comparative Example 3, theammoxidation of propane was performed for evaluating the catalystactivity, in substantially the same manner as in Example 5. The resultsare shown in Table 1.

(Evaluation of the Yield of Acrylonitrile)

Using the oxide catalyst obtained in Comparative Example 3, theammoxidation of propane was performed for evaluating the yield ofacrylonitrile, in substantially the same manner as in Example 5, exceptthat 100 hours and 200 hours after the start of the reaction, ammoniumheptamolybdate was added to the reaction system, each time in an amountof 0.1 g. The results are shown in Table 2.

COMPARATIVE EXAMPLE 9

(Evaluation of the Catalyst Activity)

Using the oxide catalyst obtained in Comparative Example 4, theammoxidation of propane was performed for evaluating the catalystactivity, in substantially the same manner as in Example 5. The resultsare shown in Table 1.

(Evaluation of the Yield of Acrylonitrile)

Using the oxide catalyst obtained in Comparative Example 4, theammoxidation of propane was performed for evaluating the yield ofacrylonitrile, in substantially the same manner as in Example 5, exceptthat the reaction was terminated 24 hours after the start of thereaction, because of too low an yield obtained. The results are shown inTable 2.

COMPARATIVE EXAMPLE 10

(Evaluation of the Catalyst Activity)

Using the oxide catalyst obtained in Comparative Example 5, theammoxidation of propane was performed for evaluating the catalystactivity, in substantially the same manner as in Example 5. The resultsare shown in Table 1.

(Evaluation of the Yield of Acrylonitrile)

Using the oxide catalyst obtained in Comparative Example 5, theammoxidation of propane was performed for evaluating the yield ofacrylonitrile, in substantially the same manner as in Example 5, exceptthat the reaction was terminated 24 hours after the start of thereaction, because of too low an yield obtained. The results are shown inTable 2.

EXAMPLE 8

(Evaluation of the Catalyst Activity)

Using the oxide catalyst obtained in Example 4, the ammoxidation ofpropane was performed for evaluating the catalyst activity, insubstantially the same manner as in Example 5. The results are shown inTable 1.

(Evaluation of the Yield of Acrylonitrile)

Using the oxide catalyst obtained in Example 4, the ammoxidation ofpropane was performed for evaluating the yield of acrylonitrile, insubstantially the same manner as in Example 5. The results are shown inTable 2.

TABLE 1 Activity (10³ hour⁻¹) Ex. 5 2.8 Ex. 6 2.7 Ex. 7 3.0 Compara. Ex.6 0.8 Compara. Ex. 7 0.1 Compara. Ex. 8 1.5 Compara. Ex. 9 1.1 Compara.Ex. 10 0.4 Ex. 8 2.6

TABLE 2 Reaction Selectivity Yield of time Conversion of for acryloni-acryloni- (hours) propane (%) trile (%) trile (%) Ex. 5 24 90.6 59.153.5 240 91.0 58.9 53.6 1300 91.1 58.7 53.5 1500 90.9 58.4 53.1 170091.0 59.0 53.7 2200 90.8 58.9 53.5 Ex. 6 24 90.5 58.8 53.2 2200 90.758.5 53.1 Ex. 7 24 91.8 58.2 53.4 2200 91.5 58.3 53.3 Compara. 24 47.322.2 10.5 Ex. 6 Compara. 24 7.7 5.2 0.4 Ex. 7 Compara. 24 71.9 47.6 34.2Ex. 8 240 68.8 47.2 32.5 Compara. 24 60.7 24.5 14.9 Ex. 9 Compara. 2427.4 29.9 8.2 Ex. 10 Ex. 8 24 88.9 58.9 52.4 2200 88.5 59.1 52.3

INDUSTRIAL APPLICABILITY

The catalyst of the present invention is advantageous not only in thatthe selectivity for and yield of the desired product in the oxidation orammoxidation are high, but also in that the catalyst exhibits only asmall lowering of the yield of the desired product even in a longreaction time. Therefore, when the catalyst of the present invention isused for performing a catalytic oxidation or ammoxidation of propane orisobutane in the gaseous phase, an unsaturated carboxylic acid or anunsaturated nitrile (namely, (meth)acrylic acid or (meth)acrylonitrile)can be produced stably in high yield for a long period of time. Further,since the catalyst of the present invention exhibits only a smalllowering of the yield with the passage of reaction time, the catalyst ofthe present invention is also advantageous in that, when a molybdenumcompound is added to the catalytic oxidation or ammoxidation reactionsystem as conventionally practiced in the art for the purpose ofmaintaining a high yield by preventing a catalyst degradation caused bythe volatilization or escaping of molybdenum from the catalyst, theamount of molybdenum compound added and the frequency of addition ofmolybdenum compound can be decreased, as compared to the case of the useof conventional catalysts, so that the reaction can be performedeconomically. In addition, the catalyst of the present invention isadvantageous in that a moderate catalyst activity can be exhibited, andhence there can be prevented problems that too large an amount ofcatalyst is required for the reaction, thus causing too heavy a load onthe reactor and that the heat of reaction generated becomes too large,rendering it impossible to effect a satisfactory heat removal from thereaction system.

1. A catalyst for use in catalytic oxidation or ammoxidation of propaneor isobutane in the gaseous phase, which comprises an oxide and a silicacarrier having supported thereon said oxide, wherein said silica carrieris present in an amount of from 20 to 60% by weight in terms of SiO₂,based on the total weight of said oxide and said silica carrier, saidoxide being represented by the following formula (1):Mo₁V_(a)Nb_(b)Sb_(c)O_(n)  (1) wherein: a, b, c and n are, respectively,the atomic ratios of vanadium (V), niobium (Nb), antimony (Sb) andoxygen (O), relative to molybdenum (Mo), wherein:0.1≦a≦1,0.01≦b≦1,0.01≦c≦1, and  n is the number of oxygen atoms required to satisfy thevalence requirements of the other component elements present, saidcatalyst having a reduction ratio of from 8 to 12% and a specificsurface area of from 7 to 30 m²/g, said reduction ratio beingrepresented by the following formula (2):reduction ratio (%)=((n ₀ −n)/n ₀)×100  (2) wherein: n is as defined forformula (1), and n₀ is the number of oxygen atoms required when theother component elements in said oxide of formula (1) respectivelyexhibit the maximum oxidation numbers of the other component elements.2. The catalyst according to claim 1, wherein a, b and c in formula (1)are as follows:0.1≦a≦0.3,0.05≦b≦0.2,0.1≦c≦0.3.
 3. The catalyst according to claim 1 or 2, wherein n₀ informula (2) is from 4 to
 5. 4. A process for producing the catalyst ofclaim 1, which comprises the steps of: providing an aqueous raw materialmixture containing compounds of molybdenum, vanadium, niobium andantimony and a source of silica, drying said aqueous raw materialmixture to thereby obtain a dried catalyst precursor, and calcining saiddried catalyst precursor under calcination conditions wherein theheating temperature of said dried catalyst precursor is continuously orintermittently elevated from a temperature which is less than 400° C. toa temperature which is in the range of from 550 to 700° C., wherein saidcalcination conditions are adjusted so that said catalyst precursorbeing calcined has a reduction ratio of from 8 to 12% when the heatingtemperature reaches 400° C., wherein said reduction ratio is as definedin claim 1, thereby obtaining a catalyst having a reduction ratio offrom 8 to 12% and a specific surface area of from 7 to 30 m²/g.
 5. Theprocess according to claim 4, wherein said aqueous raw material mixtureis obtained by mixing an aqueous mixture (A) containing compounds ofmolybdenum, vanadium and antimony with an aqueous liquid (B) containinga niobium compound.
 6. The process according to claim 5, wherein saidaqueous mixture (A) is obtained by heating, at 50° C. or more, compoundsof molybdenum, vanadium and antimony in an aqueous solvent.
 7. Theprocess according to claim 6, wherein, after said heating, hydrogenperoxide is added to said aqueous mixture (A).
 8. The process accordingto claim 7, wherein the amount of said hydrogen peroxide is such thatthe molar ratio (H₂O₂/Sb molar ratio) of said hydrogen peroxide to saidantimony compound in terms of antimony is in the range of from 0.01 to20.
 9. The process according to claim 5, wherein said aqueous liquid (B)contains a dicarboxylic acid in addition to said niobium compound,wherein the molar ratio (dicarboxylic acid/Nb molar ratio) of saiddicarboxylic acid to said niobium compound in terms of niobium is in therange of from 1 to
 4. 10. The process according to claim 5 or 9, whereinat least a part of said aqueous liquid (B) containing a niobium compoundis used in the form of a mixture thereof with hydrogen peroxide.
 11. Theprocess according to claim 10, wherein the amount of said hydrogenperoxide is such that the molar ratio (H₂O₂/Nb molar ratio) of saidhydrogen peroxide to said niobium compound in terms of niobium is in therange of from 0.5 to
 20. 12. The process according to claim 5 or 9,wherein at least a part of said aqueous liquid (B) containing a niobiumcompound is used in the form of a mixture thereof with hydrogen peroxideand an antimony compound.
 13. The process according to claim 12,wherein: the amount of said hydrogen peroxide is such that the molarratio (H₂O₂/Nb molar ratio) of said hydrogen peroxide to said niobiumcompound in terms of niobium is in the range of from 0.5 to 20, and theamount of said antimony compound mixed with the at least a part of saidaqueous liquid (B) and said hydrogen peroxide is such that the molarratio (Sb/Nb molar ratio) of said antimony compound in terms of antimonyto said niobium compound in terms of niobium is not more than
 5. 14. Theprocess according to claim 4, wherein at least a part of saidcalcination is performed in an atmosphere of an inert gas, wherein: whensaid calcination is performed in a batchwise manner, said inert gas issupplied at a flow rate of not less than 50 N liters/hour/kg of saiddried catalyst precursor, and when said calcination is performed in acontinuous manner, said inert gas is supplied at a flow rate of not lessthan 50 N liters/kg of said dried catalyst precursor.
 15. The processaccording to claim 4 or 14, wherein said calcination comprises apreliminary calcination and a final calcination, wherein saidpreliminary calcination is performed at a temperature in the range offrom 250 to 400° C. and said final calcination is performed at atemperature in the range of from 550 to 700° C.
 16. The processaccording to claim 4 or 14, wherein, during said calcination, an oxidantor a reductant is added to an atmosphere in which said calcination isperformed, so as to cause said catalyst precursor being calcined to havea reduction ratio of from 8 to 12% when the heating temperature reaches400° C.
 17. The process according to claim 16, wherein said oxidant isoxygen gas.
 18. The process according to claim 16, wherein saidreductant is ammonia.
 19. A process for producing acrylic acid ormethacrylic acid, which comprises reacting propane or isobutane withmolecular oxygen in the gaseous phase in the presence of the catalyst ofclaim
 1. 20. A process for producing acrylonitrile or methacrylonitrile,which comprises reacting propane or isobutane with ammonia and molecularoxygen in the gaseous phase in the presence of the catalyst of claim 1.